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One of the main components of the unified model of AGN (active galactic nuclei) is the circumnuclear dusty torus. The torus often obscures our view of the ''central engine" of the AGN, therefore it is important to understand its size and structure. Reverberation mapping can be used to constrain the size of the torus, and relies on the response of the torus dust emission to the variations in the driving optical/UV emission of the accretion disk. The time lag between the optical emission and the infrared response can be used to constrain the distance between the inner region of the AGN, and the IR emission region of the torus. This project focuses on a sample of 11 AGN, whose mid-IR (3.6 and 4.5 micron) light curves were observed with the Spitzer Space Telescope. The IR light curves were presented in Vazquez (2015). In this work, the optical light curves are constructed with a combination of data from three different ground-based telescopes, in the B, V, and G bands. Cross-correlation analysis has been conducted for four AGN, giving us reverberation lags for these objects. The reliability of the lags have been carefully evaluated by testing their dependence on different analysis procedures. One of these AGN is a changing-look AGN, NGC 6418, whose previous cross-correlation results have been reanalyzed. The optical-IR lag was measured both before and after a large optical flare, and was found to have increased after the flare, likely due to the sublimation of dust in the inner region of the torus. For eight of the AGN, HST images were used along with the galaxy decomposition program GALFIT to model the distribution of galaxy light and hence to remove the starlight contribution from the optical light curves. This is a necessary step in order to use the torus reverberation mapping code, TORMAC, to model the IR light curves, as the dust emission responds to the optical variations. TORMAC was used to model three of the AGN in our sample, which allows us to constrain other torus structural properties. Some of these properties include the radial cloud distribution, the radial extent of the torus, and the torus inclination angle. This dissertation produces reverberation mapping results for four AGN (KAZ 163, MRK 507, NGC 6418, and UGC 10697). micron flux with typical lags between 15-30 days. This project increases the number of AGN with mid-IR dust reverberation mapping measurements, for which there have been few previous studies. One of the AGN (NGC 6418) is found to be a rare changing-look AGN. Not only are we able to measure the size of the torus, but through the use of TORMAC we also have a more complete picture of the structure of the torus in our sample of AGN.

In the next few years, large-scale surveys with a time domain element, such as the ten-year Legacy Survey of Space and Time (LSST), will produce an immense data set to study variable and transient astronomical sources such as active galactic nuclei (AGN). AGN are located at the center of most galaxies and have five major components: a supermassive black hole (SMBH), an accretion disk, the broad-line region (BLR), a dusty circumnuclear torus, and the narrow-line region (NLR). The torus is believed to have a two-component structure: a geometrically thin circumnuclear disk and a hollow dusty cone along the polar axis. The torus absorbs the ultraviolet (UV)/optical light from the accretion disk and re-emits it in the infrared (IR). In order to study the properties of the torus, techniques such as dust reverberation mapping are needed since AGN, at large distances, cannot be resolved by direct imaging. Reverberation mapping uses the differences in the UV/optical and IR light travel times (lags) to determine the properties of the torus region. This thesis focuses on the use of the TOrus Reverberation MApping Code, TORMAC, to create simulations of the response of the IR dust emission to an input AGN light curve. Both an observed optical light curve, that of MCG-06-30-15, a narrow-line Seyfert 1 (NLS1) AGN, and a simulated AGN light curve were used as inputs. These simulations are used to determine how observing strategies (time baselines and observing cadences) and practical limitations (such as seasonal gaps, irregular sampling) affect how much information can be recovered about the properties of the torus region. In addition, I examined how various torus parameters affect the lags recovered from reverberation mapping. The work performed in this thesis will help inform the design of future optical-IR monitoring campaigns, as well as, to better understand the uncertainties in previous studies - especially those with sparse sampling and limited time duration.

A crucial breakthrough in the study of galaxy evolution occurred with the discovery that black holes are ubiquitous in galaxy nuclei, and the masses of supermassive black holes scale with several properties of the host galaxies. The interpretation is that central black holes and their host galaxies co-evolve and regulate one another’s growth. Although the role supermassive black holes play in galaxy evolution is not fully understood, their mass is a key piece of the puzzle. The most commonly adopted methods for determining supermassive black hole mass are reverberation mapping and stellar dynamical modeling. These two techniques rely on independent assumptions, observational data, and analysis procedures, and are almost always applied to different galaxies. Because of the different requirements for each technique, there are only a handful of galaxies that are candidates for both methods, and so each one plays a critical role in probing elements of the techniques themselves and identifying any systematic biases in the results. Currently, only two supermassive black holes have published masses using both methods. In this work, we present mass measurements of the supermassive black holes in two additional galaxies using stellar dynamical modeling. We first present the kinematic and photometric analysis of the lenticular galaxy NGC5273 and demonstrate that the dynamical models reveal a black hole mass of M• = [0.5 − 2.0]x10^7 M⊙. We then discuss the classical spiral galaxy NGC6814 and derive a black hole mass of M• = [0.5−1.2]x10^7 M⊙ from preliminary models. Both galaxies already have black hole masses measured with reverberation mapping, thus, we are effectively doubling the sample of galaxies for this study. The masses for NGC5273 agree, and the masses for NGC6814 are tentatively in agreement, depending on the scale factor used for the reverberation mass.

In this dissertation I present work done from 2018-2023 to investigate the growth of galaxies and supermassive black holes (SMBH) in high redshift overdensities (protoclusters) by studying the star-forming galaxy and active galactic nucleus (AGN) population in the SSA22 protocluster. I examined possible environmental sources of the enhanced star formation and AGN activity in the z = 3.09 SSA22 protocluster using Hubble WFC3 ∼ 1.6 μm observations of the SSA22 field, including new observations centered on eight X-ray selected protocluster AGN. To investigate the role of mergers in the observed AGN and star formation enhancement, quantitative and visual morphological classifications were applied to F160W images of protocluster Lyman break galaxies (LBGs) in the fields of the AGN and z ∼ 3 field LBGs in SSA22 and the GOODS-N field. No significant differences are found between the morphologies and merger fractions of protocluster and field LBGs, though this conclusion is limited by small number statistics in the protocluster. The UV-to-near-IR spectral energy distributions (SED) of F160W-detected protocluster and field LBGs were fit to characterize their stellar masses and star formation histories (SFH). These fits suggest that the mean protocluster LBG is a factor of ∼ 2 times more massive and more attenuated than the mean z ∼ 3 field LBG. These results suggest that ongoing mergers are no more common among protocluster LBGs than field LBGs, though protocluster LBGs appear to be more massive. The larger mass of the protocluster LBGs may contribute to the enhancement of SMBH mass and accretion rate in the protocluster, in turn driving the protocluster AGN enhancement. I also analyzed the physical properties of 8 X-ray selected AGN and one candidate proto- quasar system (ADF22A1) in the SSA22 protocluster by fitting their X-ray-to-IR SEDs. SFHs were recovered for 7 of these systems which are well-fit by stellar population and AGN models. The majority of the protocluster AGN systems, including ADF22A1, are consistent with obscured SMBH growth in star forming galaxies. The SFRs, SMBH accretion rates, and masses estimated from the SED fits for the 9 protocluster AGN systems are consistent with the distributions of SFR, accretion rate, SMBH masses, and stellar masses for a sample of X-ray detected AGN candidates in the Chandra Deep Fields (CDF). The ratio between the sample-averaged SSA22 SMBH mass and CDF SMBH mass is constrained to < 1.41. Furthermore, no statistically significant trends between the AGN or host galaxy properties and their location in the protocluster are found. The similarity of the protocluster and field AGN populations suggests that the observed protocluster AGN fraction enhancement may be driven by the SMBH mass distribution of the entire galaxy population, rather than only the AGN population, consistent with the interpretation of the results of the WFC3 F160W study of LBGs. The results presented here add to the existing body of knowledge on the growth of galaxies and SMBHs in protoclusters, suggesting that galaxies and their black holes grow by the same mechanisms in protoclusters and field environments, and that the observed AGN enhancement is driven instead by more frequent, secular, triggering of AGN episodes in star forming galaxies.

Blazars are a subclass of active galactic nuclei whose observable characteristics are generated by relativistic jets of high-energy plasma with trajectories closely aligned to the line of sight. This orientation results in extreme observed phenomena, such as ultraluminous emission, high amplitudes of variability, and high degrees of optical linear polarization. Furthermore, blazars are the most common extragalactic sources of γ-ray photons and have been proposed as sources of high-energy neutrinos. Long-timescale monitoring of blazars has revealed relatively quiescent states interspersed with active states featuring dramatic brightening events, with timescales ranging from months to years. New, high-time-resolution observations of blazars are revealing dramatic variability on timescales as short as several minutes, the physical drivers of which are not well understood. In this dissertation, I focus on observational signatures that can potentially identify these mechanisms. To characterize this variability, I combine observations of blazars from many telescopes, obtaining a comprehensive view of events occurring in the jets. Using the Very Long Baseline Array, I investigate moving and quasi-stationary features in the parsec-scale jets of 38 blazars. I find that the flow speeds, orientation, brightness temperatures, and opening angles of the jets in flat-spectrum radio quasars (FSRQs), BL Lacertae type objects (BL Lac objects), and radio galaxies (RGs) are statistically different, with FSRQs having the highest speeds and smallest viewing angles. Focusing on a typical source of each subclass, I characterize the optical variability observed at 2-min cadence (with the Transiting Exoplanet Survey Satellite), at gamma-ray energies (Fermi Large Area Telescope), X-ray energies (NICER, NuSTAR, and Swift satellites), and optical (ground-based instruments, especially, the Perkins telescope), including linear polarization measurements. I find that the minimum timescale of RG variability is longer than those of the FSRQ and BL Lac object, and is likely caused by changes in the accretion disk. In contrast, the variability in the FSRQ and BL Lac object can be associated with shocks, magnetic reconnections, and turbulence in the jet. The unprecedented availability of data, which promises to expand in the future, represents a new phase of observational astronomy that provides valuable information on changes in the jets of blazars.

The study of Active Galactic Nuclei formation and evolution is important in modern astronomy, as they represent an evolutionary stage in every galaxy, and thus are important to understand the formation and evolution of galaxies. However, there is not yet a full understanding of the physics and evolution of the population that emit brightly at radio wavelengths. In order to properly compare observations with theory, a large survey of radio-loud galaxies is required. In this project, I will be studying Double Radio sources Associated with Galactic Nuclei, as I wanted to learn more about them. I will be focusing on the Fanaroff-Riley types I and II in particular. I study their luminosities and find a large overlap between the two classes, moreso than previous studies, with no obvious ‘break’. I also study their alignment in the sky, and find significant alignment up to 4 degrees. This is to test claims that the position angles of radio galaxies are aligned over cosmic distances. Such an alignment could only have arisen at the time of formation which would perhaps be able to provide some insight into early formation of supermassive black holes. I will be using data from the Evolutionary Map of the Universe pilot survey that was obtained with CSIRO’s Australian Square Kilometre Array Pathfinder, as well as GAMA-23 Early Science. A survey of this size will allow study of a large sample of sources that are free from selection effects and biases of previous and older studies and will be a valuable training set for my colleagues who are developing machine learning algorithms to find and classify radio sources.

Active galactic nuclei (AGNs) are powered by accretion of material onto their central supermassive black holes and are unique in that they emit radiation across the entire electromagnetic spectrum. Spectral energy distributions (SEDs) allow for a comparison of the energy that is emitted in each wavelength regime. Our goal is to construct the most accurate multi-wavelength SEDs by reexamining the near-IR to X-ray SEDs of 23 AGNs with reverberation mapped (RM) black hole mass measurements. These SEDs will allow us to constrain measurements, such as bolometric luminosity and accretion rate and, in turn, these measurements can be used to better understand physical processes of AGNs and how they influence the host galaxy. To improve on previous studies, this set of SEDs consists of simultaneous optical/UV/X-ray data taken by XMM-Newton and is supplemented by ground-based near-IR data to constrain the red tail. Using two-dimensional decompositions of high-resolution Hubble Space Telescope images, combined with galaxy spectral templates, we constrain and remove the host galaxy contribution and focus the SEDs on the accretion powered AGN flux. We also consider the impacts of internal reddening and the effects of broad and narrow line region emission. Finally, given the highly variable nature of AGNs, we examine 4 objects from this sample with 200-300 simultaneous optical/UV/X-ray Neil Gehrels Swift Observatory observations and quantify how variability impacts the shape of the SEDs and the scatter in measurements of bolometric luminosity and accretion rate.

All but a few Active Galactic Nuclei are too distant for any single aperture telescope to resolve, yet the geometry of these objects is key to our understanding of the evolution of galaxies in the universe. To study the geometry of Active Galactic Nuclei, methods are leveraged using smaller telescopes to gain resolution rather than relying on larger and larger single aperture telescopes. Principally, these are via reverberation mapping which utilizes temporal and spectral resolution to gain spatial resolution and interferometry which sacrifices the light gathering power of large aperture telescopes for high angular resolution. To provide a tool for recovering geometric indicators of AGNs from reverberation mapping data, an image reconstructing algorithm is developed utilizing the alternating direction method of multipliers with compressed sensing regularization. This new algorithm is applied to the Arp 151 dataset from the Lick AGN Monitoring Project 2008. In addition to the reverberation mapping algorithm, the first calibrated extragalactic results for the CHARA Array are presented for NGC 4151. At the extreme detection limit for the instruments of the CHARA Array, the observational strategies utilized for successful observations and the results obtained are discussed.

The past 20 years of observations have revealed correlations between supermassive black holes (SMBHs) and large scale galaxy properties. These correlations are generally interpreted to be the signature of coevolution between galaxies and black holes, therefore accurate measurements of the properties on which these correlations are constructed is crucial for understanding galaxy-black hole evolution over cosmic time. We first present HI 21 cm spectroscopy for 31 AGN host galaxies with direct black hole mass (MBH) measurements from reverberation mapping (RM), 12 of which are the first published detections at 21 cm. We measure emission-line fluxes, widths, and recessional velocities from which we derive HI mass, gas mass (MGAS), and redshift. Combining MGAS with stellar mass (MSTARS) allows the baryonic mass (MBARY) to be estimated. We then combine the emission line measurements with Hubble Space Telescope and ground-based optical and near-infrared images to measure Tully-Fisher distances to 24 of the 31 galaxies, 14 of which are the first reported distances independent of redshift. We estimate dynamical mass (MDYN) within the HI radius, and a comparison to MBARY constrains the dark matter mass (MDM). We find significant correlations between MBH and MBARY, MDYN, and MDM, suggesting black hole-galaxy connections on scales larger than commonly considered. Finally, we present preliminary results of dynamical modeling of the broad line regions (BLRs) of five AGNs, four of which are the first reported. Direct modeling of the continuum light curve and broad emission lines yields constraints on BLR geometry and kinematics in addition to constraints on MBH that do not depend on the scale factor required by traditional RM methods. This work presents measurements of fundamental properties of RM AGNs and their host galaxies that may serve as important tools for future analyses devoted to furthering our understanding of black hole-galaxy coevolution.

Quasars are among the most luminous and massive objects in the universe. They host an actively accreting supermassive black hole (SMBH) at their centre that can power energetic phenomena that outshine the host galaxy. A substantial fraction (∼ 20 − 40%) of quasars show broad absorption-line (BAL) features in their rest-ultraviolet (UV) spectra that show clear evidence for powerful outflows originating from the central SMBHs. These outflows can carry a significant amount of energy and mass from the central SMBHs to their host galaxies. Therefore, BAL quasars are prime targets for investigating luminous quasar feedback on galaxies by which the star formation rate and the evolution of the host galaxies are thought to be controlled. SimBAL provides a novel method to study the complicated spectra of BAL quasars. Tens of thousands of BAL quasar spectra available in public archives were assumed to be impossible to analyze for more than five decades. The spectral synthesis code SimBAL our group developed can analyze even the most heavily absorbed BAL quasar spectra and provide constraints on the physical properties of the outflow as a function of velocity. Using this software, I discovered the most powerful BAL quasar outflow analyzed to date in SDSS J135246.37+423923.5. The spectral analysis showed that the outflow has the velocities reaching ∼ −38000 km/s and the outflowing gas is located near the dusty torus at ∼ 10 pc from the central SMBH. The outflow has the log kinetic luminosity log LKE ∼ 48.1 [erg/s] which exceeds the bolometric luminosity of the quasar and is energetic enough to power feedback. I performed the first systematic study of a large sample of low-redshift iron low- ionization broad absorption-line (FeLoBAL) quasars and characterized the general properties of the FeLoBAL outflows. The SimBAL analysis of 50 FeLoBAL quasar spectra revealed that the FeLoBAL gas is located at a wide range of distances (0 ≲ log R ≲ 4.4 [pc]) and has a wide range of physical properties. We discovered a new type of FeLoBAL quasar, loitering outflow objects, that are characterized by low outflow velocities and high column density winds located at log R ≲ 1 [pc] from the central SMBH. Finally, I used SimBAL to analyze a spectroscopic variability for J164653.72+243942.2 that showed a variable BAL features from an extremely-high velocity outflow (v ∼ 0.17c). The series of work presented in this dissertation demonstrates only a small fraction of scientific discoveries that can be made with SimBAL. I briefly summarize the results from the series of publications led by SimBAL collaboration that make use of the properties of the low-redshift FeLoBAL quasar outflows constrained from the SimBAL analysis. Finally, some of the planned future work using SimBAL is outlined.

Active galactic nuclei (AGN) driven feedback has been proposed to be one of the most efficient ways to quench star formation and help maintain quiescence in massive galaxies. However, direct evidence of feedback in typical quiescent galaxy populations has been restricted to only a handful of sources. This thesis presents a series of evidence that large-scale winds in a new (and relatively common) class of early-type galaxy, known as `red geysers', may represent AGN radio mode feedback in action. The work is primarily based on integral field spectroscopic data from the SDSS IV- MaNGA survey that has completed observing 10,010 galaxies in the optical wavelength at $z

Supermassive black hole binaries (SMBHBs) can lurk, often unseen, in the centers of post-merger galaxies, and pulsar timing arrays (PTAs) are rapidly approaching the sensitivities required to detect nanohertz gravitational waves (GWs) from these giant pairs. Independently, numerous electromagnetic surveys are seeking evidence of these dynamic duos’ effects on their host galaxies by searching for periodicities in time-domain observations. Combining these two methods to use multi-messenger techniques allows us to learn more about these binaries than using one messenger alone. In this thesis, we have created Bayesian methods to search for SMBHBs using electromagnetic observations of quasars and through GW emission in PTA data. By using electromagnetic observations to identify an SMBHB candidate, we gain numerous pieces of information that also define the source's GW emission, including the location of and distance to the SMBHB's host galaxy and the orbital period of the SMBHB. In our study, we developed the first multi-messenger techniques used by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), and applied them to a well-known supermassive black hole binary candidate, 3C 66B. We placed the lowest chirp mass limit to date on an SMBHB within 3C 66B of M Next, we analyzed the capabilities of Bayesian methods to search for electromagnetic signatures of these binaries in simulated time-domain data sets from next-generation surveys. We developed a Bayesian model selection technique to identify periodicities induced into a quasar light curve by the orbital motion of an SMBHB from within intrinsic red noise. We discovered that future surveys, such as the Legacy Survey of Space and Time (LSST), will identify more robust SMBHB candidates than current surveys, such as the Catalina Real-time Transient Survey (CRTS). Finally, we present the results of searches for bright continuous GWs (CWs) from individual SMBHBs in NANOGrav's 12.5-year data set. A red noise process, which could be the first signs of an emerging stochastic GW background (GWB), was previously detected for the first time in this data set. In our work, we searched for CWs alongside this common noise process for the first time in real PTA data, and developed necessary data-handling techniques which will be critical for the detection of a CW, which may come soon after the potentially imminent detection of the GWB.

It is now well established that a substantial fraction of all galaxy assembly occurs in intense bursts of star formation and black hole accretion, but the role of these two modes and how much they affect one another remains unclear. We thus investigate this in three complementary studies. In the first, we assemble a sample of 513 quasars identified by the Sloan Digital Sky Survey with detections by Herschel. These objects span a redshift range of 0 < z < 4, and their SEDs give a mean SFR of ~1000M☉/year. When comparing these SFRs to the intrinsic properties of the quasars, we find no clear connections between the quasars and the ongoing star formation events in their hosts. We then look for evidence of AGN feedback in broad absorption line (BAL) quasars, as such features are indicative of outflowing material. We find that high-ionization BAL quasars have indistinguishable properties to those of classical quasars. In our second study, which describes an iron low-ionization BAL quasar, SDSS J121441.42-000137.8, our results are again consistent with no feedback. Thus, it seems unlikely that feedback plays a dominant role in quenching star formation at the extreme SFRs seen in our BAL objects. We lastly study the host of an optically-bright quasar, SDSS J160705.16+355358.6, with evidence of an ongoing merger. We create the Point Spread Function (PSF) using a star that is in the same part of the field as our object, a method which is relatively unexplored. By subtracting the PSF, we are able to extract some of the host properties. We compare two PSF creation methods and find the empirical approach to be superior. Fits to the SEDs of the two galaxies are consistent with both falling on or above the main sequence of star formation. It is additionally plausible that these two galaxies could coalesce into a single massive quiescent galaxy by z ~ 2, and thus serve as progenitors to this class of galaxy that has proven challenging to our understanding of galaxy assembly.

This thesis presents a characterisation of the broadband behaviour of TeV gamma-ray emitting blazars that belong to the family of active galactic nuclei. Their high bolometric luminosity as well as their bright non-thermal emission up to at least TeV energies make them among the most energetic persistent sources in the universe. Blazars are also identified by large relativistic plasma jets that are aligned at a small angle to the observer’s line of sight. These jets have a size of order ∼kpc-Mpc and radiate non-thermal emission. The emission is strongly variable over the entire spectrum on all time scales and one can observe flux changes by an order of magnitude on sub-hour time scale in the most extreme cases. The origins of these phenomena are still poorly understood and the underlying mechanisms for the gamma-ray emission in blazars are still debated. These violent and extreme properties make blazars candidate sources of ultra-high-energy cosmic rays (E > 10^18 eV), whose origins are still unknown. Blazars are also potential factories of PeV neutrinos, which diffuse flux has been detected by the IceCube detector. The goal of this thesis is to provide a detailed characterisation of the broadband emission properties of blazars with the ultimate aim to understand their emission mechanisms. This work focuses on two TeV emitting blazars, belonging to the category of BL Lac type objects: 1ES 2344+514 and Mrk 421. These two objects exhibit on a temporary basis a synchrotron energy flux peaking at energies around or above 1 keV. Only a handful objects detected in the TeV band are known to exhibit such peculiar and extreme properties; they are dubbed as extreme high-frequency BL Lacs (EHBL). 1ES 2344+514 and Mrk 421 are also among the brightest and the closest TeV gamma-ray emitting blazars. Thanks to their brightness, their broadband evolution can be characterised on relatively short time scales by current instruments. Thanks to their proximity, their gamma-ray emission is not strongly absorbed by the extragalactic background light. Therefore, 1ES 2344+514 and Mrk 421 are ideal targets to better understand the most extreme phenomena observed in the jets of blazars. In order to achieve an optimal energy and temporal coverage from radio to TeV, the present work makes use of data from a large number of instruments: MAGIC, ACT, Fermi-LAT, Swift-BAT, Swift-XRT, Swift-UVOT, instruments from the Whole Earth Blazar Telescope network, NOT, KAIT, OVRO, Metsähovi and Medicina. This thesis also makes use of observations from the NuSTAR instrument, which is currently the most sensitive telescope in the hard X-ray regime (> 10 keV) and the only instrument capable of precisely measuring the flux on a sub-hour time scale at these energies. The NuSTAR data are crucial to constrain models given that within leptonic scenarios, the hard X-ray emission is expected to originate from electrons with similar energies than those responsible for the TeV emission measured by MAGIC and FACT. Based on this extensive multiwavelength coverage, several atypical results are reported and the complex behaviour of 1ES 2344+514 and Mrk 421 are characterised with a high precision. The emission is interpreted using leptonic scenarios set up for this work. The blazar 1ES 2344+514 was observed during a period of strong flux activity in summer 2016. The measured very-high-energy (VHE; E > 100 GeV) flux reached a similar level as the historical maximum. A high activity was also measured in the X-rays. The spectral properties during this peculiar event are investigated in detail. One finds that the X-ray spectrum is hard, indicating a low-energy component of the spectral energy distribution (SED) peaking around or above 10 18 Hz (∼4 keV). This value is about two orders of magnitude higher than during archival observations of 1ES 2344+514. 1ES 2344+514 showed an EHBL behaviour on a temporary basis in August 2016. The SED is interpreted with a leptonic and a proton-synchrotron scenario for the gamma-ray emission. Thanks to the wealth of multiwavelength data that has been collected, these models can be better constrained compared to previous works. One finds that both theoretical models are able to describe the SED but require a system out of equipartition by several orders of magnitude, challenging current acceleration process thought to occur in relativistic plasma jets. Mrk 421 was observed from radio to TeV over a ∼7-month period between November 2016 and June 2017. Based on the dense temporal and energy coverage, the correlation patterns of the source are characterised in order to better constrain the emission mechanisms. The VHE versus X-ray flux correlation is studied by binning the data into several sub-energy bands. The correlation is characterised up to the hard X-ray band. The fluxes are strongly correlated, in agreement with leptonic models, which predict that the X-ray and VHE fluxes are emitted by a single electron population. Nonetheless, the present work unveils that the correlation characteristics strongly vary across the sub-energy bands: the correlation ranges from a linear to a cubic trend. Over the multiwavelength campaign, several nights appear as outliers and follow different correlation patterns. They show an enhanced gamma-ray activity without a strong X-ray emission. Using simultaneous MAGIC/NuSTAR/Swift-XRT observations, this peculiar behaviour can be explained by an emitting region in the jet that is changing its size over time. The correlation between the X-ray and UV/optical emission is also studied. An anti-correlation at the significance level above 3σ is found. A hint of this pattern was measured only once until now and only at a marginal significance. The second occurrence of an anti-correlation reported in this work is an important result as it strongly supports the hypothesis that this behaviour has a real physical origin. It may be indicative of changes in the cooling and acceleration efficiencies of electrons in the jet. Finally, a short and bright VHE flare of Mrk 421 is investigated in detail. Interestingly, this VHE flare is identified by an absence of clear X-ray flux increase. This particular day appears as an evident outlier in the VHE versus X-ray correlation study. Using time-resolved SEDs and a leptonic modelling, this work proposes that such a behaviour may be caused by the appearance of a second electron population (which adds up to the electron population responsible for the quiescent state) with a narrow energy distribution located in a compact blob.

There is overwhelming evidence for a link between supermassive black holes (BHs) and their host galaxies, yet the physical mechanisms behind the connection are poorly understood. Discerning between possible co-evolutionary scenarios requires precise measurements of BH masses and host properties across cosmic time. BH mass measurements outside the local universe require bright active galactic nuclei (AGNs), but the AGN light overwhelms the light of the host, making host property measurements difficult. Further, BH mass estimates rely on the unresolvable broad emission line region (BLR), and the unknown nature of the BLR limits their precision.In this dissertation, I use a unique combination of gravitational lensing and reverberation mapping techniques to tackle both issues. First, I present a semi-supervised classification approach for identifying rare lensed quasars based solely on photometry in wide-field surveys. Lenses are powerful tools for measuring host properties, but studies are limited by the small sample size, so discovery efforts are crucial. Using this technique, I successfully discovered three new lenses in the Sloan Digital Sky Survey, with several other candidates awaiting further follow-up. This approach is flexible and can easily be modified to target specific types of lenses as well as lenses in surveys covering different portions of the sky.Next, I use a modeling framework to measure precise BH masses for seven AGNs and infer the structural and kinematic properties of the Hβ BLR. This nearly doubles the sample size of modeled BLRs, allowing for a correlation analysis between BLR properties and AGN observables that can be used to improve BH mass measurements for all AGNs. I then model Hβ, CIV, and Lyα in NGC 5548 using data from 85 Hubble Space Telescope orbits and multiple ground-based telescopes to understand the similarities and differences between the optical and UV BLRs. High-z BH mass measurements rely on UV lines, but our current understanding of the UV BLR is severely lacking, so understanding their connection is critical for decreasing high-z BH mass uncertainties. Finally, I tie the previous projects together with the most precise high-z BH mass measurement to date, using data from a 4.5 year monitoring campaign of a strongly lensed quasar at z=2.805. Accounting for gravitational time delays, the campaign exceeds 6 years in duration. Understanding the co-evolution of BHs and their hosts will require measurements of this precision at high-z, and this research demonstrates that gravitational lensing makes this level of precision obtainable.

Mergers play a vital role in galaxy evolution, having the potential to trigger Active Galactic Nucleus (AGN) activity, star formation, or changes in morphology. In this work, we investigate whether galaxy mergers have an effect on AGN activity in the galaxies involved. We used a visual classification scheme to classify the morphologies of nearly 50,000 galaxies and identify galaxy mergers in the CANDELS fields, a Multi-Cycle Treasury Program using the Hubble Space Telescope. We use IR and X-ray selection criteria to identify IR and X-ray AGN, and matched controls to both our AGN subsample and merger subsample. We investigated the fraction of mergers that host AGN, as well as the fraction of AGN that are found in merging systems, to study the AGN-merger connection from multiple angles. We also investigated the properties of mergers, such as the mass ratios of early-stage mergers, and their star forming properties. We find that galaxies involved in a merger or interaction are more likely than non-merging galaxies to host AGN. The fraction of merging systems that host AGN is 0.034±0.002,compared to an AGN fraction of 0.023±0.001 in non-merging systems. We also find that IR AGN are more likely to be found in merging systems, with a merger fraction of 0.353+0.021−0.019, compared to a merger fraction of 0.225+0.007−0.006 in control galaxies without AGN activity. This is likely because these are more dusty and obscured, and are expected to be observed earlier in the merger process, when merger signatures are more visible. The AGN detected in either the X-ray or both the IR and X-ray are even less likely to be found in merging systems, as the merger signatures are expected to fade later in the merging process. These results show that mergers play an important role in driving AGN activity.

"Quasar-mode feedback" occurs when momentum and energy from the environment of accreting supermassive black hole couple to the host galaxy. One mechanism for such a coupling is by high-velocity (up to ~0.2c) quasar-driven ionized outflows, appearing as blue-shifted absorption and emission lines in quasar spectra. Given enough energy and momentum, these outflows are capable of affecting the evolution of their host galaxies. This dissertation presents the studies of emission and absorption quasar outflows from different perspectives. (1). By conducting large broad absorption line (BAL) quasar surveys in both Sloan Digital Sky Survey and Very Large Telescopes (VLT), we determined various physics properties of quasar absorption outflows, e.g., the electron number density ((ne), the distance of outflows to the central quasar (), and the kinetic energy carried by the outflow (̇k). We demonstrated that half of the typical BAL outflows are situated at > 100 pc, i.e., having the potential to affect the host galaxies. (2). Our group carried out a Hubble Space Telescope program (PI: Arav) for studying the outflows in the Extreme-UV, collaborating with Dr. Gerard Kriss from Space Telescope Science Institute (STScI). We developed a novel method to fit the multitude of quasar absorption troughs efficiently and accurately. We have identified the most energetic quasar-driven outflows on record and discovered the largest acceleration and velocity-shift for a quasar absorption outflow. (3). By using the VLT data, Xu led the project to study the relationships between BAL outflows and emission line outflows. We found possible connections between these two types of quasar outflows, e.g., the luminosity of the [III λ5007 emission profile decreases with increasing ne derived from the BAL outflow in the same quasar. These findings are consistent with BAL and emission outflows being different manifestations of the same wind, and the observed relationships are likely a reflection of the outflow density distribution.

The center of every large galaxy is believed to host a supermassive black hole (SMBH). In some of these galaxies, the nuclear region is extremely luminous and far outshines the entire stellar population. Called active galactic nuclei (AGN), these central sources are equally fascinating and perplexing. Due to their immense distance and small size, modern observation techniques are unable to resolve the structure of these regions. However, the polarization of scattered light carries the imprint of the geometry and kinematics of both the sources and the scattering medium. Here, we present a study of the broad H$\alpha$ lines in a sample of 46 AGN. Following the work of Smith et al. 2002, 2004, and 2005, we show supporting evidence for the proposed two-component scattering model of AGN using the largest sample of spectropolarimetric observations of broad-line AGN assembled to date. Objects of differing polarization classifications (equatorial or polar) are shown to have significant differences in their distributions of both total flux and polarized flux inter-percentile velocity values. Furthermore, clear distinctions between these classifications are found in polarization position angle and core polarization percentage. These results are supported by Kolmogorov-Smirnov tests between the classifications and are in accordance with those suggested by previous studies. We also explore the validity of a spectropolarimetric method of estimating SMBH masses. In general, SMBH mass estimates require information on the inclination of the object in the sky, but we can avoid requiring this information by using polarized inter-percentile velocity measurements instead. Using this method, we achieve estimates on the order of 10^6 to 10^8 solar masses -- estimations consistent with both the expected values and those in the literature.

Although the first supermassive black hole (SMBH) was observed over 100 years ago, the details regarding how they form, evolve, and impact their surroundings remain active topics of research. In fact, only over the past 5 years has the existence of black holes been confirmed via the advent of detectors such as the Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and the Event Horizon Telescope (EHT). From an observational perspective, although all massive galaxies are thought to harbor nuclear SMBHs, the observability is dependent on the accretion activity. The activity of accreting SMBHs, or Active Galactic Nuclei (AGN), is expected to evolve with time and is likely a function of the ability to efficiently channel gas inflow to the galactic center. Many black hole feeding mechanisms exist, such as gravitational instabilities in galaxies that are barred or nucleated, gas dynamic processes involving multiple black holes, and major galaxy interactions. To further complicate the matter, it is possible that more than one mechanism plays a role at any given time. However, analyzing which SMBHs are active, and why, is vital to understanding various feedback processes and how the growth of a SMBH and its host galaxy are tied. In this dissertation, I present various analyses that are all focused on studying the activity of SMBHs in various environments. Specifically, I have studied a population of AGN in nucleated galaxies to analyze how nuclear star clusters affect SMBH activity (Chapter 2), and a binary AGN candidate (two SMBHs that are gravitationally bound; Chapter 3) that did not appear to be undergoing any major merger. However, the majority of my dissertation focuses on detecting and analyzing AGN pairs, (``dual AGN", if the SMBHs are not yet gravitationally bound). Despite the importance of dual AGN to wide-ranging astrophysical fields such as galaxy formation and gravitational waves, the rate of dual AGN has yet to be accurately measured. Yet, the rate of dual AGN can inform us of the role galaxy mergers play in triggering AGN, timescales for post-merger SMBHs to sink to the center of the potential well, as well as merger-related feedback physics. Dual AGN that are widely separated relative to the instrument PSF and have near unity flux ratios are easy to identify, however dual AGN with small separations and/or flux ratios can only be distinguished from a single AGN with advanced statistical analysis. As a result, very few dual AGN have been confirmed. Thus, I've developed a tool called BAYMAX (Bayesian Analysis of Multiple AGN in X-rays), that quantitatively evaluates whether a given source in a Chandra observation is composed of a single or multiple point sources, using a Bayesian framework. With BAYMAX, I am methodically expanding the known population of multiple AGN systems (Chapters 5 and 6), while learning more about their preferential environments via multi-wavelength (Optical, IR, and X-ray) analyses.

It has long been assumed that Active Galactic Nuclei (AGN) reside at the centers of their host galaxies, but is this really true? A galaxy merger is expected to lead to the formation of a supermassive black hole (SMBH) binary, which can shrink through dynamical processes until it eventually coalesces through the emission of gravitational waves. Numerical relativity simulations show that, depending on the initial spin-orbit configuration of the binary, the merged SMBH receives a gravitational recoil kick that may reach several 1000 km s−1. The kick causes the merged SMBH to oscillate for up to ∼ 1 Gyr in the gravitational potential well of the galaxy, during which, the recoiling SMBH may be observed as a ‘displaced’ AGN. Displacements ∼ 10-100pc may be expected even in nearby elliptical galaxies and can be measured as spatial offsets in high resolution optical/infrared images. In this dissertation, I present the results of a study of ∼100 nearby elliptical galaxies, that host AGN, using Hubble Space Telescope archival and new optical/infrared images, to analyze spatial offsets between the photo- center and AGN position. Evidence for significant spatial offsets has been found in about 20% of the sample of which six are considered to be robust displacements (∼5 - 40pc). Three other galaxies display a dual nucleus structure. These results are discussed in the context of the gravitational recoil hypothesis and alternative displacement mechanisms, including acceleration by radio jets, are considered. A second study described in this dissertation focusses on the luminous quasar E1821+643 which has previously been identified as an SMBH recoil candidate based on the large Doppler shifts of emission lines attributed to the retained gas. Follow-up spectroastrometry revealed a spatial displacement between the nucleus and the gas emitting the [OIII] lines. Hubble Space Telescope (HST) images were used to map the distribution of narrow-line emission on sub-arcsecond spatial scales which show that the [OIII] emission is intrinsically asymmetric. Spectroastrometry simulations reveal that the asymmetric [OIII] emission can account for only ∼25% of the observed displacement, which strengthens the case for a recoiling SMBH.

The mass of a supermassive black hole (MBH) is a fundamental property that can be obtained through observational methods. Constraining MBH through multiple methods for an individual galaxy is important for verifying the accuracy of different techniques, and for investigating the assumptions inherent in each method. However, there exist only a few galaxies where multiple MBH determination techniques can be applied. NGC 4151 is one of these rare galaxies for which multiple methods can be used, stellar and gas dynamical modeling because of its proximity and reverberation mapping because of its active accretion. In this work, we reduced the integral field unit spectroscopy of the nucleus of NGC 4151 from Onken et al., observed in the H band with Gemini North NIFS, improving the process itself as well as the analysis of the spatially-resolved spectra. We also improved on the methods for constraining the line of sight velocity distribution as a function of position within the nucleus. Stellar dynamical modeling was then performed over a range of choices of MBH and mass-to-light ratio, as well as the de-projected luminosity density for various inclinations. The best reproduction of the observed kinematics and luminosity density was found with MBH = 2.44 +/- 0.16 x 10^7 MSun and mass-to-light ratio = 0.318 +/- 0.003 MSun/LSun and an inclination of 45 degrees. This measurement falls within the range of values in the literature; it is below the reverberation mapping mass of Bentz et al. (3.59 +0.45 -0.37 x 10^7 MSun) and above the reverberation mapping mass of De Rosa et al. (1.97 +/- 0.04 x 10^7 MSun), and within the uncertainties on both the gas dynamical modeling mass of Hicks & Malkan (3.0 +0.8 -2.2 x 10^7 MSun) and the stellar dynamical modeling mass of Onken et al. (3.60 +/- 1.10 x 10^7 MSun). This work also represents a preparatory step in the application of a new bar-optimized stellar dynamical modeling code.

This dissertation consists of two parts. In the first part, we focus on studying the nuclear outbursts in the centers of galaxies and their nature in order to better understand the behavior of central Super Massive Black Holes (SMBHs) and their interaction with the surrounding environment, and to better understand the accretion disk structure. Nuclear outbursts can be better understood by studying the changes in the broad emission lines and the underlying continuum. We quantify the properties of these nuclear outbursts using multi-wavelength observations including optical, ultraviolet, and X-rays from MDM Observatory, the Sloan Digital Sky Survey, Swift, and Magellan. Some of these nuclear outbursts are linked to Tidal Disruption Events (TDEs) and nuclear supernovae (SNs), while a number of these events are proposed to be a rare phenomenon called “changing-look” Active Galactic Nuclei (AGN). These types of AGNs have been observed to optically transition from type 1 to type 2 and vice versa on timescales of months to years, where broad emission lines such as Hα and Hβ appeared or disappeared followed by an increase or decrease in the continuum light. We investigate two transient events called PS1-13cbe and PS1-10cdq that were observed during outburst by the PS1 survey in 2013 and 2010, respectively. We investigate TDE, SN, and AGN activity as the three possible scenarios for the nature of these events. In the case of PS1- 3cbe, we conclude that “changing-look” behavior caused by thermal accretion disk instabilities is the most plausible explanation for the outburst. However, in the case of PS1-10cdq, we favor the tidal disruption scenario because of the structure of the lightcurve and spectral evolution. In the second part of this dissertation, we focus on galaxy morphology prediction using a newly designed neural network called “Capsule Networks”. We automate the process of morphology prediction and eliminate the need for feature engineering and heavy data prepossessing prior to classification. We also reconstruct the galaxy images while preserving the brightness structure of the galaxies. This study provides one of the possible solutions for classifying objects for the era of large sky surveys where the amount of data for galaxies will dramatically increase and automated methods will play a very crucial role.

This thesis studies actively accreting supermassive black holes (SMBH) known as quasars and their impact on the formation and evolution of massive galaxies. Theoretical work predicts that quasars are responsible for regulating the growth of the SMBH and its host galaxy. Winds driven by radiation pressure from the accretion disk and quasar jets are predicted to remove a substantial amount of gas from the host galaxy. Massive galaxies are observed to form the majority of their stars when the Universe was only 3.5 billion years old. During this epoch, the most massive SMBH already grew a substantial fraction of their mass. The SMBH is predicted to regulate the growth of the host galaxy during this co-evolutionary phase.We have undertaken an extensive adaptive optics (AO) assisted integral field spectroscopy (IFS) survey of distant quasar host galaxies when the Universe was only 2.2-4.6 Gyr old. We conducted observations using the 10-meter W.M. Keck Observatory optical and infrared telescope. The survey aims to measure the gas-phase properties of quasar host galaxies such as metallicities, star formation rates, photoionization mechanisms, and search for galaxy-wide outflows potentially driven by the quasar. We detect outflows in all objects on scales from

This thesis presents a joint analysis of the rest-frame ultraviolet (UV) luminosity functions of continuum-selected star-forming galaxies and galaxies dominated by active galactic nuclei (AGNs) at z ~ 4. These 3,740 z ~ 4 galaxies are selected from broad-band imaging in nine photometric bands over 18 deg² in the Spitzer/HETDEX Exploratory Large Area Survey (SHELA) field. The large area and moderate depth of our survey provide a unique view of the intersection between the bright end of the galaxy UV luminosity function (M [subscript AB] < -22) and the faint end of the AGN UV luminosity function. We do not separate AGN-dominated galaxies from star-formation-dominated galaxies, but rather fit both luminosity functions simultaneously. These functions are best fit with a double power-law (DPL) for both the galaxy and AGN components, where the galaxy bright-end slope has a power-law index of -3:80 ± 0.10, and the corresponding AGN faint-end slope is α [subscript AGN] = -1.49 +0.30/-0.21. We cannot rule out a Schechter-like exponential decline for the galaxy UV luminosity function, and in this scenario the AGN luminosity function has a steeper faint-end slope of -2.08 +0.18/-0.11. Comparison of our galaxy luminosity function results with a representative cosmological model of galaxy formation suggests that the molecular gas depletion time must be shorter, implying that star formation is more efficient in bright galaxies at z = 4 than at the present day. If the galaxy luminosity function does indeed have a power-law shape at the bright end, the implied ionizing emissivity from AGNs is not inconsistent with previous observations. However, if the underlying galaxy distribution is Schechter, it implies a significantly higher ionizing emissivity from AGNs at this epoch.

Pair halos would be observational evidence of interactions between electromagnetic cascades in the intergalactic medium and non-zero magnetic fields. If detected, they would represent a crucial probe into the evolution of our universe and potentially new physics at very early cosmological times. However, they are also expected to be extremely faint. In this thesis, we take a three-stage approach to examining pair halos. We determine the halo's spectral, spatial, and temporal morphology by building a detailed physical simulator of the pair halo process. We then use this understanding to build detection methods which rely on the unique spatial and temporal characteristics of the halo and its progenitor active galactic nucleus (AGN). Finally, we test one of these methods on data from the Fermi Large Area Telescope, determine its sensitivity to different halos, and predict the performance improvements needed to make detections with future instruments.

Radiation pressure dominated accretion discs may have turbulent velocities that exceed the electron thermal velocities. Bulk Comptonization by the turbulence may therefore dominate over thermal Comptonization in determining the emergent spectrum. We discuss how to self-consistently resolve and interpret this effect in calculations of spectra of radiation MHD simulations. In particular, we show that this effect is dominated by radiation viscous dissipation and can be treated as thermal Comptonization with an equivalent temperature. We investigate whether bulk Comptonization may provide a physical basis for warm Comptonization models of the soft X-ray excess in AGN. We characterize our results with temperatures and optical depths to make contact with other models of this component. We show that bulk Comptonization shifts the Wien tail to higher energy and lowers the gas temperature, broadening the spectrum. More generally, we model the dependence of this effect on a wide range of fundamental accretion disc parameters, such as mass, luminosity, radius, spin, inner boundary condition, and the alpha parameter. Because our model connects bulk Comptonization to one dimensional vertical structure temperature profiles in a physically intuitive way, it will be useful for understanding this effect in future simulations run in new regimes. We also develop a global Monte Carlo code to study this effect in global radiation MHD simulations. This code can be used more broadly to compare global simulations with observed systems, and in particular to investigate whether magnetically dominated discs can explain why observed high Eddington accretion discs appear to be thermally stable.

I have investigated the energy output of active galactic nuclei (AGN) in order to understand how these objects evolve and the impact they may have on host galaxies. First, I looked at a sample of 96 AGN at redshifts $z \sim 2, 3,$ and $4$ which have imaging and thus luminosity measurements in the $griz$ and $JHK$ observed wavebands. For these galaxies, I have co-epochal data across those bands which accounted for variability in AGN luminosity. I used the luminosity measurements in the five bands to construct spectral energy distributions (SED) in the emitted optical-UV bands for each AGN. I compared the SED to assumptions previously made about quasars and looked for correlations between SED and other AGN and galaxy properties. Second, I used spectra of the broad line region (BLR) of Type 1 AGN to estimate the mass of the central supermassive black hole ($M_{BH}$). I found a sample of Type 1 AGN that reside in spiral galaxies in order to explore the relationship between $M_{BH}$ and pitch angle ($\phi$), a measurement of how tightly wound the spiral arms are. Type 1 AGN offer a method to estimate $M_{BH}$ at higher redshift than previous studies of the $M_{BH}$-$\phi$ relation. I was able to look at the evolution in the $M_{BH}$-$\phi$ relation which has implications for galaxy formation as well as AGN feedback.

Supermassive black holes (SMBH) are now thought to exist at the center nearly all massive galaxies. Not only are they thought to be ubiquitous, but it was also discovered nearly two decades ago that the mass of these SMBHs correlate strongly with properties of their host galaxies including bulge stellar velocity dispersion (MBH-sigma) and bulge luminosity (MBH-Lbulge). This correlation was not expected due to the tiny size of the SMBH gravitational sphere of influence compared to the size of the host galaxy, and imply a connection between the two, but this connection is still not well-understood. One step toward understanding this connection is to accurately measure the masses of these black holes. Two of the most common direct SMBH mass measurement techniques are stellar dynamical modeling (SDM), which generally only applies to quiescent galaxies, and reverberation mapping (RM), which can only be applied to active galactic nuclei (AGN) that exhibit broadened emission lines. Due to the unknown geometry of the region that produces these broad lines, the whole RM sample of black hole masses generally needs to be multiplied by a constant called the f-factor to bring it into agreement with the SDM sample on the MBH-sigma relation. It is unknown how well this f-factor, being a population average, applies to individual RM masses. It would therefore be useful to measure an SMBH mass with both methods simultaneously to test whether they produce the same black hole mass. However, because the RM and SDM techniques usually apply to galaxies that are not possible for both, this has only been attempted twice before (NGC 3227 and NGC 4151). The purpose of this dissertation is to apply SDM to the SMBH at the center of NGC 6814 for which there already exists an RM mass. This makes it only the third broad-lined AGN for which an SDM mass has been derived. In order to perform SDM accurately, the distance to the galaxy needs to be well-constrained as the error in the SDM mass scales linearly with distance. Because no adequate distance measurements already exist, the first half of this dissertation is devoted to deriving a Cepheid distance to NGC 6814 from V- and I-band HST WFC3 time series photometry. We measure the distance to NGC 6814 to be 17.54 +1.44/-1.33 Mpc. In the second half, we incorporate that distance measurement into our stellar dynamical modeling on Gemini NIFS+Altair IFU data of NGC 6814's central 1.55''x1.55''. We derive a mass of 1.19 +37.57/-1.17 x108 solar masses, and best fit mass-to-light ratio of 0.948 +0.032/-0.208 in solar units. This mass is nearly an order of magnitude larger than the RM mass but has a 3-sigma range spanning nearly three orders of magnitude. We describe possible reasons for our larger-than-expected mass value, such as the existence of a bar, which would not be well-modeled by an axisymmetric dynamical code. Finally, we describe future steps that may be taken to better constrain the mass, such as creating more models to further explore parameter space.

Supermassive Black Holes (BHs; typically with masses greater than 10^5 solar masses) are ubiquitous in the cores of all massive galaxies (stellar masses greater than 10^10 solar masses). However, BHs in dwarf galaxies (stellar masses less than 10^9.5 solar masses) are more difficult to detect, and thus have remained relatively elusive. Theoretical work suggests that clues to BH formation and growth can be found in present day dwarf galaxies, making this an important population to study. Searching for signs of BH accretion is one way of identifying BHs in dwarf galaxies more distant than the Local Group. Until recently, only a handful of dwarf galaxies had been identified with actively accreting central BHs, or Active Galactic Nuclei (AGNs). However, with the advent of large-scale surveys, it has been possible to search for signs of BH accretion in samples comprised of tens of thousands of galaxies. Recent works have identified roughly two hundred dwarf galaxies with AGN signatures, making it possible to begin to carry out demographic studies. This thesis describes an in-depth, multi-wavelength characterization of this relatively unexplored population. We determine which observational AGN identifiers used for massive galaxies also apply for dwarf galaxies, study the accretion properties of confirmed AGNs, and explore relationships between BH mass and host galaxy properties in these systems. We also present evidence for an active ~50, 000 solar mass BH in the center of the dwarf galaxy RGG 118, which at the time of writing, is the smallest BH yet reported in a galaxy center.

Active Galactic Nucleus (AGN) produce rich, luminescent spectra across the electro- magnetic spectrum. This work focuses on topics in the ultraviolet region, where lie the broad absorption line outflows innate to Broad Absorption Line Quasi-Stellar Objects (BALQSOs) and the Fe II bump, characteristic of strong Fe emitters. We find the incidence of P V absorption in AGN to be ∼ 10% and only observe P V absorption in Broad Absorption Lines (BALs), never in narrow lines. Our troughs exhibit envelope behavior in vmax versus Luminosity, as previously observed, with P V detected troughs forming the upper edge of the envelope. The spectral and photometric variabilities in the low luminosity, narrow line Seyfert 1 galaxy WPVS007 are consistent with the BAL wind ablating off the edge of a variable scale height torus. WPVS 007 underwent the first ultraviolet occultation yet observed for an AGN. The differences in ultraviolet spectral shape of the Fe emission shape observed in strong Fe emitting AGN is attributable to higher excitation Fe II and excess Fe III in PHL1811-like objects, but photoionization is not sufficient to fit the observed emission.

This thesis presents numerical calculations designed to understand aspects of the feeding of, and feedback from, super-massive black holes at the centers of galaxies. The first portion of the thesis describes the development of radiative transfer tools used to address these problems. I present a description of how the Monte Carlo technique can be used to solve the radiative transfer equation, and I demonstrate a coupling of the transfer solution with the equations of hydrodynamics and statistical equilibrium. I next present two major applications of these ideas. The first is to quantify the effects of radiative feedback in active galactic nuclei in the form of radiation pressure on dust at the center of a gas-rich galaxy. The second is a calculation of the spectral energy distributions and optical spectral line strengths emitted during the tidal disruption of a star by a massive black hole. In so doing, I help to answer a number of puzzling questions relating to such disruptions, such as whether the lack of hydrogen emission in their spectra can be the result of radiative transfer effects rather than a lack of hydrogen in the disrupted star.

We study two classes of object to gain a better understanding of the evolution of Active Galactic Nuclei (AGN): Infrared-Faint Radio Sources (IFRSs) and Gigahertz Peaked Spectrum (GPS) / Compact Steep Spectrum (CSS) sources. IFRSs are a recently discovered rare class of object, which were found to be strong in the radio but undetectable in extremely sensitive infrared observations from the Spitzer Space Telescope, even in stacked images with σ < 1μJy. IFRSs were found to exhibit a relatively high sky density, and were thought to represent AGN at z > 3. Therefore, IFRSs may significantly increase the number of known high-redshift galaxies. However, their non-detections in the optical and infrared prevented confirmation of their nature. Previous studies of IFRSs focused on very sensitive observations of a few small regions of the sky, and the largest sample consisted of 55 IFRSs. However, we follow the strategy of combining radio data with IR and optical data for a large region of the sky. Using these data, we discover a population of >1300 brighter IFRSs which are, for the first time, reliably detected in the infrared and optical. We present the first spectroscopic redshifts of IFRSs and show that the brightest IFRSs are at z > 2. Furthermore, we rule out that IFRSs are Star Forming Galaxies, hotspots, lobes or misidentifications. We find the first X-ray counterparts of IFRSs, and increase the number of known polarised IFRSs five-fold. We present an analysis of their radio spectra and show that IFRSs consist of GPS, CSS and ultra-steep-spectrum sources. We follow up >50 of these using VLBI observations, and confirm the AGN status of IFRSs. We conclude that IFRSs represent a new population of high-redshift radio galaxies, which, for the faintest IFRSs, may have redshifts as high as z = 7 and consist of a few hundred thousand objects across the μJy sky. GPS and CSS sources are compact radio sources with a convex radio spectrum. They are widely thought to represent young and evolving radio galaxies that have recently launched their jets. However, good evidence exists in individual cases that GPS and CSS sources are one of the following: 1) frustrated by interactions with dense gas and dust in their environment; 2) prematurely dying radio sources; 3) recurrent radio galaxies. Their convex spectrum is generally thought to be caused by Synchrotron Self Absorption (SSA), an internal process in which the same population of electrons is responsible for the synchrotron emission and self-absorption. However, recent studies have shown that the convex spectrum may be caused by Free-Free Absorption (FFA), an external process in which an inhomogeneous screen absorbs the synchrotron emission. The majority of GPS and CSS samples consist of Jy-level and therefore, high-luminosity sources. VLBI images show that GPS and CSS sources typically have double-lobed, edge-brightened morphologies on mas scales, appearing as scaled down versions of Fanaroff-Riley Class II (FR II) galaxies. Recently, two low-luminosity GPS sources were found to have jet-brightened morphologies, which appeared as scaled down versions of Fanaroff-Riley Class I (FR I) galaxies. From this, it was proposed that there exists a morphology-luminosity break analogous to the FR I/II break and that low-luminosity GPS and CSS sources are the compact counterparts of FR I galaxies. However, this hypothesis remains unconfirmed, since very few samples of low-luminosity GPS and CSS sources exist. We have observed the faintest population of GPS and CSS sources to date, consisting ⇠150 sources, many of which are low-luminosity. We use high-resolution radio observations to determine their linear size, resolve their jets and observe their small-scale morphology. We combine these data with a large number of radio observations at other frequencies to model their radio spectra using SSA and FFA models. In particular, we use very low frequency observations that have only recently become available to constrain their spectral peaks. We follow up eight of the most compact sources with VLBI and detect six of them. We find that our GPS and CSS sources are well modelled by an inhomogeneous FFA model (hereafter ‘FFA’). Furthermore, we find a number of very compact GPS and CSS sources that are inconsistent with SSA theory. We show that a single inhomogeneous SSA model fits poorly to the majority of radio spectra, predicting far too steep a slope below the peak. We resolve all of the sources with VLBI and derive their kinematic ages based on the jet sizes. Even when assuming more complex SSA models, we derive magnetic field strengths several orders of magnitude too high for one source. A few sources are well modelled by an FFA model in which the inhomogeneous absorption is dominated by clouds of high density, consistent with the frustration hypothesis. However, the majority of sources are well modelled by FFA models with low-density clouds. These models suggest that an inhomogeneous and clumpy medium surrounds the sources, implying they may undergo recurrent activity. Furthermore, the spectral model of one CSS source suggests it is a prematurely dying radio galaxy whose jets have been switched off for ∽600 years. However, we find no evidence of restarted radio galaxies within our high resolution observations. The kinematic and spectral ages we derive are consistent with the hypothesis that GPS and CSS sources are young and evolving. We find tentative evidence that at mJy-levels, the fraction of CSS sources is smaller than the fraction estimated for Jy-level sources. This may be accounted for as a selection effect or if a smaller fraction of mJy-level GPS sources evolve into CSS sources as compared to the Jy-level GPS sources. We find a few GPS sources with low luminosities, which we will follow up with VLBI to test whether they are the compact counterparts of FR I galaxies. We conclude that, despite being historically favoured, single inhomogeneous SSA is not the dominant form of absorption amongst a large fraction of GPS and CSS sources. We find that FFA provides a good model for the majority of the spectra with observable turnovers, suggesting an inhomogeneous and clumpy ambient medium. Furthermore, we conclude that the majority of our GPS and CSS sources are young and evolving and may undergo recurrent activity over small time scales. We conclude that a very small fraction of GPS and CSS sources consists of frustrated, dying or restarted radio galaxies.

Measuring the masses of supermassive black holes in active galactic nuclei (AGN) allows us to trace their evolution over cosmic time and understand how black holes coevolve with their host galaxies. We present a new technique to measure black hole masses and constrain the structure of the broad line region in AGN using reverberation mapping data. We begin by developing a simply parameterized phenomenological model of the broad line region geometry and dynamics and apply this model to high-quality reverberation mapping data for six AGN from the Lick AGN Monitoring Project 2008 and 2011 datasets. The results of this analysis provide the most precise AGN black hole masses from reverberation mapping to date and the first detailed constraints on the geometry and dynamics of the broad line region emission. Specifically, we find that the shape of the broad line region is generally a close to face-on thick disk with preferential emission from the far side, and that the dynamics range from inflow to near-circular orbits. In addition, we present photometric AGN light curves using image subtraction for the Lick AGN Monitoring Project 2011 dataset as a first step towards modeling the broad line region in a larger sample of AGN.

Recent research has lead to the possible discovery of a new class of gamma-ray emitting ac- tive galactic nuclei (AGN). These objects – the very radio-loud narrow-line Seyfert 1 galaxies (NLSy1s)– demonstrate observational features suggesting that they are similar to blazars. One of the key characteristics of blazars is the presence of high-amplitude optical microvariability. While this phenomenon has been investigated in individual objects, no study of the intra-night variability of radio-loud NLSy1s as a class has previously been available. This dissertation presents a sys- tematic search for optical variability in a sample of 33 radio-loud NLSy1s. It was found that 26 objects demonstrated microvariations. However, only 9 objects did so with duty cycles comparable to blazars, and only 7 of these 9 objects — J0706+3901, J0849+5108, J0948+0022, J1246+0238, PKS 1502+036, J1644+2619, and IRAS 20181-2244 — demonstrated microvariability at compa- rable amplitudes. Two objects stand out as exceptional sources. J0849+5108 was found to have a duty cycle of ~90% and was observed to undergo an enormous 4-magnitude optical flare in a two-month time span. The object has not been reported to have undergone such an event since 1975. The second object, J0948+0022, is the class prototype. High cadence data indicates that J0948+0022 has a remarkably rapid doubling time scale of ~40 minutes, and it was seen to vary by over 0.9 magnitudes within an individual night. Attempts to correlate microvariability to radio loudness, gamma-ray loudness, and other parameters were largely unsuccessful. However, it was found that only radio-loud NLSy1s that were detected at gamma-ray energies demonstrated microvariability at blazar-like duty cycles. Additionally, an analysis of the frequency of microvariations at various amplitudes suggests that the sample of radio-loud NLSy1s presented in this study share a parent population identical to low energy peaked BL Lac-type (LBL) blazars. This is in agreement with the work of astronomers such as Abdo et al. 2009, who have created spectral energy distributions for a few radio-loud NLSy1s and found them to resemble those of LBLs. Blazar-like variability was found in multiple objects with radio loudnesses of log(R) < 2, suggesting that even moderately radio-loud NLSy1s may be blazar-like objects.

The recent discovery of a new population of Active Galactic Nuclei (AGN) – the Radio Loud Narrow Line Seyfert 1 (RL NLS1) galaxies – at g-ray energies by Fermi has prompted intense interest among researchers, as evidence mounts that these objects may in fact compose a new class of blazars. If RL NLS1s are indeed a new class of blazars, or at least analogous to them, then the detection of certain blazar-like properties would be expected. These properties include significant variability at all wavelengths across the electromagnetic spectrum and on timescales from minutes to years, significant & variable polarization in the radio and optical regimes, significant & variable high-energy emissions (especially in the g-ray regime), and a double-peaked structure of their spectral energy distributions. This dissertation seeks to characterize several of these properties for RL NLS1s as a class. These include the degree to which these objects are polarized and the variability of this property, the detection and characterization of these sources at g-ray energies with the Large Area Telescope aboard the Fermi spacecraft, and the degree to which these properties are interdependent. A photopolarimetric survey (the first of its kind for this class of objects) and g-ray monitoring program were conducted by the author in order to obtain the data necessary for this project. The measurements obtained via these observations are used to characterize this interesting class of objects with respect to a sample of blazars. In general, it was found that the polarizations of these objects fall between radio-quiet NLS1s and FSRQ-type blazars, and were most similar to HBL-type blazars. The 7 RL NLS1s in this sample that had been detected above a Test Statistic (TS) of 25 by Fermi/LAT were most similar to FSRQs, while 9 objects detected in the interval 9 < TS < 25 shared several properties with HBLs. Two RL NLS1s - J1443+4725 and J1644+2619 - are identified as high-confidence (TS > 25) g-ray sources for the first time, bringing the total number of members of this class firmly detected at g-ray energies to 8. The gamma-ray spectra of RL NLS1s are similar to FSRQs, though some have steeper spectra.

Active Galactic Nuclei (AGNs) are the most mystic objects in the universe. They are usually very far away from our Galaxy, which means that they are ancient objects. They are also luminous and have unique features in their spectra. Studying AGNs helps understanding the early universe and the evolution of galaxies. This Dissertation aims to research the structure of AGNs and the cooling function in the AGNs environment. I first investigate what optical/ultraviolet spectroscopic features would be produced by Broad-line Region (BLR) clouds crossing our line of sight to the accretion disk, the source of the optical/UV continuum. This research, prompted by recent X-ray observations, suggests that single cloud has little effect on the optical/UV spectrum. However, an ensemble of clouds produces a strong distinctive feature between the Lyman limit and Lyα. The extent of these features indicates the line-of-sight covering factor of clouds and may explain the ubiquitous AGN spectral break around 1100Å. I next study, considering the physical parameters of AGNs, how the gas cooling function changes at high temperature (T > 104 K) over a wide range of density (nH < 1012 cm−3) and metallicity (Z < 30Z⊙). I find that both density and metallicity change the ionization status of the gas. I provide numerical cooling functions by describing the total cooling as a sum of four parts: that due to H&He, the heavy elements, electron-electron bremsstrahlung, and grains. Finally, I also provide a function giving the electron fraction, which can be used to convert the cooling function into a cooling rate. Last, I extend the cooling-function study to the seldom-explored low-temperature range (T < 104 K). For primordial gas, gas lacking elements heavier than B, I find that radiative attachment and Compton recoil are important cooling processes when the gas kinetic temperature is lower than the temperature of the cosmic microwave background. I also find that collisional de-excitation of HD and H2 is not important above 1000K unlike claims of previous studies. For the dust-free solar case, we identify water as the dominant coolant in high density-environments. We also analyze the parameter ranges where metal, metal molecules, or all molecules, dominate the total cooling. We provide the density, above which the metal or metal molecules become the dominant coolants, as a function of temperature and metallicity. For the ISM case, with dust and depleted abundances, we find that dust does not directly cool the gas. Rather, dust modifies he cooling by affecting the chemistral balance. Similar to the high-temperature case, I also provide numerical cooling data.

Active galactic nuclei (AGN) ubiquitously show outflows. It is now widely recognized that these outflows are key components in the evolution of super-massive black holes and their host galaxies. As important as these outflows are, we still lack sufficient understanding of their structure and energetics. The majority of the work presented in this thesis involved photoionization modeling of AGN outflows along with analysis of density diagnostics in order to determine the distances and energetics of observed outflows. The main findings of these analyses are that 1) outflows are often at distances of hundreds to thousands of parsecs from the central supermassive black hole and 2) quasars outflows can be sufficiently powerful to provide feedback in galactic evolution scenarios. We also find in some cases that the recombination timescales of metal ions are long compared with the flux variability timescales. The large distances we find provide a challenge to current outflow models. For example, these outflows cannot be connected with an accretion disk surrounding the supermassive black hole as assumed in some models. Furthermore, the outflows may be out of equilibrium as we find in Mrk 509. In this case, a thorough understanding of time-dependent photoionization effects is necessary. In this thesis, I include early steps toward understanding time-dependent photoionization as well as ionization studies of accretion disk winds. The main results of these theoretical studies is that 1) the appearance of multiple ionization components in an outflow can be an artifact of the incorrect assumption that the outflow is in ionization equilibrium and 2) the shielding gas required in accretion-disk-wind models should have a clear signature in UV spectra, but none has been observed to date.

In this dissertation, I present the results of work to improve measurements of black hole masses (MBH) and investigate the structure of the broad line region (BLR) in active galactic nuclei (AGNs), which are key tracers of galaxy formation and evolution. I address these issues by carrying out high-cadence reverberation mapping experiments on several targets using the telescopes at MDM Observatory on Kitt Peak and by further testing the relationship between MBH and host-galaxy velocity dispersion (sigma) as a calibrator for the MBH scale.Reverberation mapping can be employed for two specific purposes: 1) Measurement of MBH in AGNs and 2) Determination of BLR dynamics. We obtained improved reverberation measurements of the radius of the BLR and MBH for the object PG 2130+099 and performed a re-analysis of previous data. Reverberation data were also collected at several observatories over a 140-day span in 2010. We obtained high sampling-rate light curves for five objects, from which we have measured the average radius of the H-Beta-emitting region and calculated MBH. Our new measurements substantially improve previous measurements for these objects. We also measure the lag in the He II emission line relative to the optical continuum in Mrk 335, which is the first robust lag measurement for a high-ionization line in a narrow-line Seyfert 1 galaxy.We also recovered velocity-resolved reverberation lags for all five AGNs and velocity-delay maps for four. In all four objects, velocity-delay maps show evidence for gravitational infall and in some cases Keplerian motion. These maps constitute a large increase in the number of objects for which we have resolved velocity-delay maps and provide evidence supporting the reliability of reverberation-based MBH measurements.Finally, I discuss the results of an investigation with the Gemini North 8-m telescope to obtain sigma measurements in high-mass quasars. I report successful measurements for four objects that improve the quality of two measurements near the high-mass end of the distribution and add two new objects to the sample. We update the entire AGN MBH-sigma sample and obtain a new estimate of the virial factor used to calibrate all MBH measurements in AGNs.

I have investigated the nature of a subset of active galactic nuclei (AGN) which show double peaks in their characteristic optical and ultraviolet emission lines. I have performed this investigation through studies of the broad emission line regions (BLRs), which are produced less than 1 pc from the central supermassive black hole (SMBH), and the narrow emission line regions (NLRs), which originate at larger (kpc) distances. The BLR studies consist of detailed line modeling of two individual quasars with double-peaked broad emission line profiles. The modeling suggests there are two primary interpretations of the complex broad line profiles. The first possiblity is line emission from the surface of an asymmetric and/or non-uniform accretion disk of a low-accretion rate AGN; these sources are known as double-peaked emitters and account for only about 3% of the quasar population. The second possibility is line emission from the BLRs of two actively accreting SMBHs in a close (pc) binary system. Such binaries are an inevitable outcome following the merger of two galaxies. The NLR studies consist of three separate projects. The first is an analysis of a candidate AGN pair (dual AGN), with a separation of 5.5 kpc, in a galaxy at a redshift of z=1.175; this scenario would be the result of a galaxy merger and represent the stage prior to the formation of a binary SMBH. The second is the identification of similar candidate dual AGN sources in a systematic study of quasars at redshifts z=0.8-1.6. The final project analyzes follow-up long-slit spectroscopy of two quasars found through the systematic search which are most likely to host AGN-driven outflows. Overall, the combined results for these NLR studies show that a significant fraction of the double-peaked narrow emission lines are produced by AGN-driven outflows. Furthermore, diagnostics based upon ionization potentials can effectively select sources most likely to possess these outflows, and those which are more likely to host dual AGN. Finally, follow-up observations in the form of long-slit spectroscopy and high-resolution imaging have proven effective at distinguishing between those scenarios.

The parsec-scale radio properties of blazars detected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope have been investigated using observations with the Very Long Baseline Array (VLBA). Comparisons between LAT and non-LAT detected samples were made using both archival and contemporaneous data. In total, 244 sources were used in the LAT-detected sample. This very large, radio flux-limited sample of active galactic nuclei (AGN) provides insights into the mechanism that produces strong gamma-ray emission. It has been found that LAT-detected BL Lac objects are very similar to the non-LAT BL Lac objects in most properties, although LAT BL Lac objects may have longer jets. The LAT flat spectrum radio quasars (FSRQs) are significantly different from non-LAT FSRQs and are likely extreme members of the FSRQ population. Archival radio data indicated that there was no significant correlation between radio flux density and gamma-ray flux, especially at lower flux levels. However, contemporaneous observations showed a strong correlation. Most of the differences between the LAT and non-LAT populations are related to the cores of the sources, indicating that the gamma-ray emission may originate near the base of the jets (i.e., within a few pc of the central engine). There is some indication that LAT-detected sources may have larger jet opening angles than the non-LAT sources. Strong core polarization is significantly more common among the LAT sources, suggesting that gamma-ray emission is related to strong, uniform magnetic fields at the base of the jets of the blazars. Observations of sources in two epochs indicate that core fractional polarization was higher when the objects were detected by the LAT. The low-synchrotron peaked (LSP) BL Lac object sample shows indications of contamination by FSRQs which happen to have undetectable emission lines. There is evidence that the LSP BL Lac objects are more strongly beamed than the rest of the BL Lac object population.

This thesis is a theoretical study of supermassive black holes (SMBHs) in merging galaxies. We consider the dynamics that govern inspiralling SMBH pairs and gravitational-wave (GW) recoiling SMBHs, as well as the fueling of active galactic nuclei (AGN) during galaxy mergers. In particular, we focus on the observable signatures that could distinguish dual or recoiling AGN from those in isolated galaxies, and we explore the implications of these events for the coordinated evolution of SMBHs and galaxies. In the second and third chapters, semi-analytical models for GW-recoiling SMBHs are developed. The second chapter illustrates that bound recoiling SMBHs may have long wandering timescales and that recoil events can self-regulate SMBH growth. In the third chapter, we study the evolution of recoiling SMBHs in evolving, gaseous merger remnants. We find that the presence of gas greatly influences recoiling SMBH trajectories and may partially suppress even large recoil kicks in some cases. We also show that kinematically- and spatially-offset AGN can have substantial lifetimes for a wide range in kick speeds. Finally, this chapter illustrates that GW recoil influences the observed SMBH-galaxy relations as well as central star formation in the merger remnant. In the fourth chapter we turn our attention to inspiralling SMBH pairs with kiloparsec-scale separations. We use a novel approach to model the narrow-line emission from these SMBH pairs, in order to understand their relationship to observations of double-peaked narrow-line AGN. Our results indicate that double-peaked narrow-line AGN often arise from gas kinematics rather than from dual SMBH motion, but that the latter are a generic, short-lived phase of SMBH inspiral in gaseous mergers. We identify several diagnostics that could aid in distinguishing the true AGN pairs in the double-peaked sample. Finally, the fifth chapter examines a particular galaxy that exhibits signatures of both a recoiling AGN and an AGN pair. Applying methods developed throughout this thesis, we design models for both scenarios that are well-matched to the available data. Currently, neither possibility can be excluded for this object, but our models constrain the most relevant parameters for etermining its nature and for the design of future observations.

This dissertation examines two broad topics -- emission line properties of active galactic nuclei (AGN) and the effect of hands-on, integrated science courses on student understanding of astronomy. To investigate trends in overall properties of emission lines in AGN, we apply principal component analysis (PCA) to the fluxes in the H [beta] - (O III) region of a sample of 9046 spectroscopically-identified broad-line AGN from the Sloan Digital Sky Survey (SDSS) Data Release 5 with a redshift range of 0.1 < z < 0.56. After performing independent spectral PCA on subsets defined effectively by their (O III) equivalent width (EW), we find only the weakest (O III) objects retain the optical Fe II - (O III) anticorrelation and the correlation of EW[subscript O III] with H [beta] linewidth that have previously been found in high-luminosity AGN. The objects with strongest EW[subscript O III] do not differ from the entire data set significantly in other spectral and derived properties, such as luminosity, redshift, emission line shapes, Eddington ratio, continuum slope, and radio properties. However, our findings are consistent with previous suggestions that (O III) emission is primarily a function of covering factor of the narrow-line region. To investigate the other side of the Fe II - (O III) anticorrelation, we examine the effect of changes in the gas-phase abundance of Fe on observed variation in Fe II. Using AGN spectra from the SDSS in the redshift range of 0.2 < z < 0.35, we measure the Fe/Ne abundance of the narrow-line region (NLR) using the (Fe VII)/(Ne V) line intensity ratio. We find no significant difference in the abundance of Fe relative to Ne in the NLR as a function of Fe II/H [beta]. However, the (N II)/(S II) ratio increases by a factor of 2 with increasing Fe II strength. This indicates a trend in N/S abundance ratio, and by implication in the overall metallicity of the NLR gas, with increasing Fe II strength. We propose that the wide range of Fe II strength in AGN largely results from the selective depletion of Fe into grains in the low ionization portion of the broad-line region. We utilize photoionization models to show that the strength of the optical Fe II lines varies almost linearly with gas-phase Fe abundance, while the ultraviolet Fe II strength varies more weakly, as seen observationally. After examining the emission line properties of large samples of fairly typical AGN, we investigated the newly expanded regime of low-mass AGN (M[subscript BH] [less than or approximately equal to] 10⁶ M[subscript sun]) with respect to their emission line properties at a smaller scale. We utilize the high spectral resolution and small aperture of our Keck data of 27 low-mass AGN, taken with the Echellette Spectrograph and Imager, to isolate the NLRs of these low-mass black holes. Some of these low-luminosity objects plausibly represent examples of the low-metallicity AGN described by Groves et al. (2006), based on their (N II)/H[alpha] ratios and their consistency with the Kewley & Ellison (2008) mass-metallicity relation. We also find that these low-mass AGN have steeper UV continuum slopes than more-massive AGN based on their He II/H[beta] ratio. Overall, NLR emission lines in these low-mass AGN exhibit trends similar to those seen in AGN with higher-mass BHs, such as increasing blueshifts and broadening with increasing ionization potential. Additionally, we see evidence of an intermediate line region whose intensity correlates with L/L[subscript Edd] in these objects, as seen in higher-mass AGN. We highlight the interesting trend that, at least in these low-mass BHs, the (O III) EW is highest in symmetric NLR lines with no blue wing. This trend of increasing (O III) EW with line symmetry could be explained by a high covering factor of lower ionization gas in the NLR. We also investigate effective methods for teaching astronomy and connections between astronomical topics in student learning and understanding. After developing the curriculum for a hands-on, learner-centered astronomy course (Hands-on-Science, hereafter HoS) aimed at pre-service elementary teachers, we measure student performance in HoS compared to traditional, large lecture courses (hereafter Astro101). We utilize distractor-driven multiple choice assessments in order to quantitatively assess student understanding and evaluate the persistence or correction of common misconceptions in astronomy. We find that for the topics included in the HoS curriculum, HoS students have a higher average post-test score, and higher normalized gains, than the Astro101 students. We cannot pinpoint the exact cause of this student achievement because of the multitude of nontraditional practices incorporated into the HoS implementation. Increased time-on-task, a classroom environment structured around student discussion, or focus on conceptual understanding could each be key factors in the high achievement of HoS students. We conclude that the HoS students are better prepared in astronomy for their future careers as elementary school teachers by HoS courses than they would have been in traditional, introductory astronomy courses. When we compare directly between topics covered in both HoS and Astro101, we find that HoS students have normalized gains that are a factor of 2-4 higher than those of Astro101 students. Therefore, we conclude that curricula similar to the HoS approach would benefit Astro101 students as well, particularly for topics which are most impacted by the HoS method, such as Moon phases and seasons. Lastly, a PCA of the changes in HoS student scores reveals that there is very little systematic student variation apart from the trends apparent in the mean changes in the sample. Thus, we do not find groupings of questions that some subsets of students systematically learn more readily than others. Another way to interpret this result is that the HoS curriculum and methodology indiscriminately help all kinds of pre-service elementary teachers, despite presumptive differences in their own learning styles and strengths.

All classes of Active Galactic Nuclei (AGN) are fundamentally powered by accretion of gas onto a supermassive black hole. The process converts the potential energy of the infalling matter to X-ray and ultraviolet (UV) radiation, releasing up to several 1012 solar luminosities. Observations show that the accreting "central engines" in AGN are surrounded by dusty matter. The dust occupies a "torus" around the AGN which is comprised of discrete clumps. If the AGN radiation is propagating through the torus on its way to an observer, it will be heavily re-processed by the dust, i.e. converted from UV to infrared (IR) wavelengths. Much of the information about the input radiation is lost in this conversion process while an imprint of the dusty torus is left in the released IR photons. Our group was the first to formulate a consistent treatment of radiative transfer in a clumpy medium an important improvement over simpler models with smooth dust distributions previously used by researchers. Our code CLUMPY computes spectral energy distributions (SED) for any set of model parameters values. Fitting these models to observed AGN SEDs allows us to determine important quantities, such as the torus size, the spatial distribution of clumps, the torus covering factor, or the intrinsic AGN luminosity. Detailed modeling also permits us to study the complex behavior of certain spectral features. IR radiative transfer introduces degeneracies to the solution space: different parameter values can yield similar SEDs. The geometry of the torus further exacerbates the problem. Knowing the amount of parameter degeneracy present in our models is important for quantifying the confidence in data fits. When matching the models to observed SEDs we must employ modern statistical methods. In my research I use Bayesian statistics to determine the likely ranges of parameter values. I have developed all tools required for fitting observed SEDs with our large model database: the latest implementation of CLUMPY, the fit algorithms, the Markov Chain Monte Carlo sampler, and the Bayesian estimator. In collaboration with observing groups we have applied our methods to a multitude of real-life AGN.

In the framework of active galactic nuclei (AGNs), a galaxy’s supermassive black hole is surrounded by a dusty torus whose clumpy configuration allows for either direct or obscured views toward the central engine. Viewing AGNs from different angles gives rise to a variety of AGN classifications; for example, the generic Type 1 AGN class requires the detection of optically broad emission lines, which arise from quickly moving material within the torus, whereas Type 2 AGNs lack these observations. While these viewing angles are not directly observable, synthetic torus models generated with CLUMPY provide a means to determine them along with other parameters that describe the nature and characteristics of the torus in general. Employing CLUMPY models with mid-infrared spectroscopic observations of a large sample of both Type 1 and Type 2 AGNs allows us to acquire a further understanding of the clumpy torus structure and its viewing angles.

The classical "cooling flow" model historically associated with "cool core" clusters of galaxies fails in the absence of an external, non-gravitational heating mechanism needed to offset catastrophic radiative losses of the X-ray bright intracluster medium (ICM). Numerous proposed solutions exist, including feedback from active galactic nuclei (AGN), which may elegantly calibrate fundamental relationships such as the coupled co-evolution of black holes and the stellar component of their host galaxies. AGN feedback cannot completely offset cooling at all times, however, as the brightest cluster galaxies (BCGs) in cool core clusters harbor extensive warm (∼104 K) and cold (10 < T < 104 K) gas reservoirs whose physical properties are regulated by ongoing star formation and an unknown, non-stellar heating mechanism. We present a doctoral thesis broadly related to these issues, particularly as they pertain to cooling flows, the triggering of AGN activity, and the associated energetic feedback that may play a critical role in heating the ambient environment on tens to hundreds of kiloparsec scales. We begin with a summary of the relevant background material, and in Chapter 2 we present a multiwavelength study of effervescent AGN heating in the cool core cluster Abell 2597. Previously unpublished Chandra X-ray data show the central regions of the hot intracluster medium (ICM) to be highly anisotropic on the scale of the BCG, permeated by a network of kpc-scale X-ray cavities, the largest of which is cospatial in projection with extended 330 MHz radio emission. We present spectral maps of projected, modeled gas properties fit to the X-ray data. The X-ray temperature map reveals two discrete, ``hard-edged'' structures, including a ∼15 kpc ``cold filament'' and an arc of hot gas which in projection borders the inner edge of the large X-ray cavity. We interpret the latter in the context of the effervescent AGN heating model, in which cavity enthalpy is thermalized as the ambient keV gas rushes to refill the wake of the buoyant bubble. The hot arc revealed in the temperature map may be one of the first instances in which ICM/ISM heating by AGN feedback is directly observed. The ∼15 kpc soft excess filament, part of which is cospatial with extended 1.3 GHz radio emission, may be associated with dredge-up of low entropy gas by the propagating radio source. Results from our study of the hot X-ray gas are framed in the context of inferred young stellar component ages associated with the central emission line nebula in the BCG. We find that inferred ages of the young stellar component are both younger and older than the inferred ages of the X-ray cavities, suggesting that low levels of star formation have managed to persist amid the AGN feedback-driven excavation of the X-ray cavity network. In Chapter 3 we present Hubble Space Telescope far-ultraviolet imaging of seven BCGs in cool core clusters selected on the basis of elevated star formation rates. We find that even at low levels, star formation provides a dominant contribution to the ionizing photon reservoir required to power the observed luminosities of the emission line nebula. Weak, compact radio sources are observed in each of these seven BCGs. The combination of higher SFR and lower radio power is consistent with a scenario wherein a low state of AGN feedback allows for increased residual condensation from the ambient X-ray atmosphere, accounting for the elevated star formation rates. In Chapter 4 we present a comparison study of episodic star formation and AGN activity in the giant radio galaxy 3C 236, which is not associated with a cluster. We find that an episodic AGN/starburst connection can be fostered by a non-steady transport of gas to the nucleus. These results are then compared with Abell 2597, enabling a better understanding of the roles that may be played by cooling flows vs. mergers and hot vs. cold accretion modes in depositing the gaseous reservoirs that fuel both star formation and AGN activity. In Chapter 5 we broaden the context of the thesis with a search for high redshift Fanaroff-Riley class I radio galaxies, which may act as observable "beacons" for assembling protoclusters. Probing the epoch of cluster assembly will be critical to a better understanding of the evolution of the cool core phenomenon and the history of cluster entropy regulation in general. The relative inability of X-ray cluster selection techniques to extend to these redshifts necessitates alternative detection methods, one of which we describe in this thesis. Finally, in Chapter 6 we discuss the main conclusions of this thesis, which can be summarized as follows: (1) AGN feedback is real, and likely plays a dominant role in regulating the pathway of entropy loss from hot ambient medium to cold gas to star formation; (2) AGN feedback does not establish an impassable "entropy floor" below which gas cannot cool; and (3) star formation plays an important role in determining the temperature and ionization of the warm (∼104 K) and cold (10 < T < 104 K) gas phases in brightest cluster galaxies.

Active Galactic Nuclei (AGN) are some of the most extreme objects in the universe. They output copious amounts of energy spanning the entire electromagnetic spectrum. There are many different subclasses of AGN depending on your viewing angle. Blazars, viewing down the relativistic jet, are the most variable class of AGN known. They exhibit extreme variability in all wavelengths on timescales as short as minutes. In this thesis I will consider the extreme faintness of 3C279 with respect to the long-term light curve as well as recent observations of microvariability. I am able to confirm small amplitude events using simultaneous observations from two telescopes and cross correlation analysis. Transitory quasi-periodic oscillations are observed during two of the nights with confirmed microvariability.

We present the complete results from two, high sampling-rate,multi-month, spectrophotometric reverberation mapping campaignsundertaken to obtain either new or improved Hβ reverberation lagmeasurements for several relatively low-luminosity active galacticnuclei (AGNs). We have reliably measured the time delay betweenvariations in the continuum and Hβ emission line in seven localSeyfert 1 galaxies. These measurements are used to calculate the massof the supermassive black hole at the center of each of these AGNs. Weplace our results in context to the most current calibration of thebroad-line region (BLR) RBLR-L relationship, where ourresults remove many outliers and significantly reduce the scatter at thelow-luminosity end of this relationship.A detailed analysis of the data from our high sampling rate, multi-monthreverberation mapping campaign in 2007 reveals that the Hβ emissionregion within the BLRs of several nearby AGNs exhibit a variety ofkinematic behaviors. Through a velocity-resolved reverberation analysisof the broad Hβ emission-line flux variations in our sample, wereconstruct velocity-resolved kinematic signals for our entire sampleand clearly see evidence for outflowing, infalling, and virialized BLRgas motions in NGC 3227, NGC 3516, and NGC 5548, respectively.Finally, we explore the nature of systematic errors that can arise inmeasurements of black hole masses from single-epoch spectra of AGNs byutilizing the many epochs available for NGC 5548 and PG1229+204 fromreverberation mapping databases. In particular, we examine systematicsdue to AGN variability, contamination due to constant spectralcomponents (i.e., narrow lines and host galaxy flux), data quality(i.e., signal-to-noise ratio, S/N), and blending of spectral features.We investigate the effect that each of these systematics has on theprecision and accuracy of single-epoch masses calculated from twocommonly-used line-width measures by comparing these results to recentreverberation mapping studies. We then present an error budget whichsummarizes the minimum observable uncertainties as well as the amount ofadditional scatter and/or systematic offset that can be expected fromthe individual sources of error investigated.

Relations between supermassive black holes (SMBHs) and their hostgalaxies are now well known, but several questions remain: Do allgalaxies harbor SMBHs? Do correlations between BH mass and host galaxyproperties extend to lower mass BHs and all galaxy types? Is thegalactic bulge or the dark matter halo the defining component for thenuclear BH? Answering these questions requires a study of low massSMBHs, in particular those that reside in the latest-type spiralgalaxies. While the presence of an active galactic nucleus (AGN)provides certain proof of the existence of an SMBH, galaxies that arenot previously known to host AGNs may nevertheless have SMBHs at theircenters. In other words, a galaxy may appear quiescent when in realitythere is an accreting SMBH, if the accretion level is low enough. Thisthesis presents a search for such SMBHs by looking for the presence oflow-level nuclear activity, as evidenced primarily by their x-rayemission, in a well-defined sample of nearby, optically quiescentspiral galaxies. This work demonstrates that traditional methods ofidentifying AGNs, developed over the course of studying luminous(> 1042 erg/s) AGNs, are inadequate for the population of lowluminosity AGNs found in nearby galaxies, and develops the techniquesthat must be used instead. These techniques are then applied to anx-ray survey of nearby, face-on spiral galaxies. The survey includesnew snapshot observations of 37 galaxies, which are combined witharchival data for a further 18 galaxies. Where available,multi-wavelength data are used to help distinguish AGNs from othertypes of x-ray sources. These observations show the power of x-rayobservations in detecting hidden AGNs, and also address the questionof the prevalence of SMBHs in spiral galaxies that do not havebulges. This study has uncovered 14 previously unknown AGNs and strongAGN candidates, including two in galaxies of type Sd and Sdm. If thelatter are confirmed as bona fide AGNs they will be only the fourthand fifth AGNs known to exist in bulgeless galaxies.