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A small subset of extragalactic double radio sources, termed HYbrid MOrpholgy Radio Sources (HYMORS), is distinguished by a very unusual, hybrid morphology in terms of Fanaroff–Riley (FR) classification. In HYMORS, one radio lobe appears edge-darkened (FR I), while the other shows a well-defined emission peak near its outer edge (edge-brightened, FR II). Such sources are rare, but critical for constraining the mechanism responsible for FR dichotomy, a widely debated issue in extragalactic astrophysics. Here, we highlight the need for caution in assigning FR type, in view of some upcoming observational campaigns to confirm HYMORS among the candidates. To illustrate this, we highlight the cases of three radio sources, which have been perceived to be HYMORS, including the radio galaxy 0500 + 630 (4C + 63.07), which has been claimed to be a good, original example of a HYMORS, with a FR I western lobe and a FR II eastern lobe marked by a prominent terminal hot spot. However, its recent VLASS map at 3 GHz has revealed that the western lobe actually extends much farther out than reported and terminates in a well-defined emission peak. This implies that the source is a regular FR II radio galaxy and not a HYMORS. We also provide a brief perspective of the HYMORS phenomenon and underscore the need to confirm a FR I classification by ruling out additional FR II characteristics, such as an inward lobe-widening and spectral steepening, as well as a lack of prominent radio jet within the lobe. [ABSTRACT FROM AUTHOR]

This article gives a brief historical introduction and reviews our current understanding of jets in radio galaxies and quasars from an observational perspective, with an emphasis on observations at radio wavelengths. Recent results on the Fanaroff–Riley (FR) classification scheme, and the nature of radio structures and jets in the FR classes as well as in high-excitation and low-excitation radio galaxies are summarized. The collimation and propagation of jets from nuclear sub-pc to hundreds of kpc scales from both observational and theoretical works have been discussed. The jets exhibit evidence of interaction with a clumpy interstellar medium, especially in young radio sources, and could trigger both star formation as well as suppress star formation depending on the physical conditions. Observational evidence for such interactions and jet feedback which have profound implications our understanding of galaxy evolution have been presented. Recurrent jet activity which has been seen over a wide range of projected linear size and time scales has been discussed. This review article concludes with a brief discussion of unresolved questions on jets which new telescopes should help address. [ABSTRACT FROM AUTHOR]

The Event Horizon Telescope (EHT) project has recently published a high-resolution picture of the Centaurus A galaxy core, where a supermassive black hole (SMBH) is supposed to be located. This picture has been accompanied with interpretation difficulties. In particular, the expected central SMBH and its accretion disk are not detected, the conical forward plasma jet seems hollow and its bended shape is difficult to explain. We argue that this image could in fact reveal the existence of two SMBHs, instead of one. Within this alternative interpretation, the black holes and their disks become visible on the image, the curved jet shapes may more easily be explained and the jets display no dark spine. The putative SMBH binary system shows a projected separation distance of ∼0.4-0.7 milliparsec. This is 700 times narrower than the NGC 7674 SMBH binary, so far the visually-identified system with the shortest such distance (0.35 pc). The orbital period can be inferred to lie in the 101 to 10−1 yr magnitude range. The image suggests a double helical jet structure that, if real, would support the lower end of this estimation range. If confirmed by future observations, this close SMBH binary in Centaurus A will be of great interest as a testing ground for SMBH binary dynamics models and plasma jet studies. [ABSTRACT FROM AUTHOR]

The next two decades are expected to open the door to the first coincident detections of electromagnetic (EM) and gravitational-wave (GW) signatures associated with massive black-hole (MBH) binaries heading for coalescence. These detections will launch a new era of multimessenger astrophysics by expanding this growing field to the low-frequency GW regime and will provide an unprecedented understanding of the evolution of MBHs and galaxies. They will also constitute fundamentally new probes of cosmology and would enable unique tests of gravity. The aim of this Living Review is to provide an introduction to this research topic by presenting a summary of key findings, physical processes and ideas pertaining to EM counterparts to MBH mergers as they are known at the time of this writing. We review current observational evidence for close MBH binaries, discuss relevant physical processes and timescales, and summarize the possible EM counterparts to GWs in the precursor, coalescence, and afterglow stages of a MBH merger. We also describe open questions and discuss future prospects in this dynamic and quick-paced research area. [ABSTRACT FROM AUTHOR]

Probing the physics of the accretion flow around active galactic nuclei (AGN) is crucial to understanding their emission mechanisms as well as being able to constrain the geometrical and variability properties of the different regions around them. The soft X-ray excess – usually observed below ∼ 2 keV in excess of the dominant X-ray powerlaw continuum – is one prominent feature that is commonly seen in type 1 Seyfert AGN and therefore readily provides a useful diagnostic of the accretion flow mechanism around these systems. NGC 5940 is a Seyfert 1 AGN which reveals strong, prominent soft X-ray excess below ∼ 2 keV as seen in both its XMM-Newton and Swift observations. Model fit to the data revealed that this feature could be equally well explained by the ionised partial covering, the thermal Comptonisation and the blurred reflection models. Although the other models cannot be decisively ruled out with the data at hand, the lack of significant broad iron K α as well as any significant emission/absorption line features in the reflection grating spectrometer (RGS) data tend to favour the thermal Comptonisation origin for the soft X-ray excess in NGC 5940. [ABSTRACT FROM AUTHOR]

We present a detailed timing and spectral study of an extremely variable narrow-line Seyfert 1 galaxy NGC 4748 using observations in the year 2017 and 2014 performed with AstroSat and XMM-Newton, respectively. Both observations show extremely variable soft and hard X-ray emission that are correlated with each other. In the 2014 data set, the source retains its general behaviour of “softer when brighter” while the 2017 observation exhibits a “harder when brighter” nature. Such changing behaviour is rare in AGNs and is usually observed in the black hole binary systems. The “harder when brighter” is confirmed with the anti-correlation between the photon index and the 0.3–10 keV power-law flux. This suggests a possible change in the accretion mode from standard to the advection-dominated flow. Additionally, both the observations show soft X-ray excess below 2 keV over the power-law continuum. This excess was fitted with a single or multiple blackbody component(s). The origin of soft excess during the 2017 observation is likely due to the cool Comptonization as the photon index changes with time. On the other hand, the broad iron line and delayed UV emission during the 2014 observation strongly suggest that X-ray illumination onto the accretion disk and reflection and reprocessing play a significant role in this AGN. [ABSTRACT FROM AUTHOR]

Accretion onto black holes is an efficient mechanism in converting the gas mass-energy into energetic outputs as radiation, wind and jet. Tidal disruption events, in which stars are tidally torn apart and then accreted onto supermassive black holes, offer unique opportunities of studying the accretion physics as well as the wind and jet launching physics across different accretion regimes. In this review, we systematically describe and discuss the models that have been developed to study the accretion flows and jets in tidal disruption events. A good knowledge of these physics is not only needed for understanding the emissions of the observed events, but also crucial for probing the general relativistic space-time around black holes and the demographics of supermassive black holes via tidal disruption events. [ABSTRACT FROM AUTHOR]

Radio observations of tidal disruption events (TDEs) probe material ejected by the disruption of stars by supermassive black holes (SMBHs), uniquely tracing the formation and evolution of jets and outflows, revealing details of the disruption hydrodynamics, and illuminating the environments around previously-dormant SMBHs. To date, observations reveal a surprisingly diverse population. A small fraction of TDEs (at most a few percent) have been observed to produce radio-luminous mildly relativistic jets. The remainder of the population are radio quiet, producing less luminous jets, non-relativistic outflows or, possibly, no radio emission at all. Here, we review the radio observations that have been made of TDEs to date and discuss possible explanations for their properties, focusing on detected sources and, in particular, on the two best-studied events: Sw J1644+57 and ASASSN-14li. We also discuss what we have learned about the host galaxies of TDEs from radio observations and review constraints on the rates of bright and faint radio outflows in TDEs. Upcoming X-ray, optical, near-IR, and radio surveys will greatly expand the sample of TDEs, and technological advances open the exciting possibility of discovering a sample of TDEs in the radio band unbiased by host galaxy extinction. [ABSTRACT FROM AUTHOR]

Numerical simulations have historically played a major role in understanding the hydrodynamics of the tidal disruption process. Given the complexity of the geometry of the system, the challenges posed by the problem have indeed stimulated much work on the numerical side. Smoothed Particles Hydrodynamics methods, for example, have seen their very first applications in the context of tidal disruption and still play a major role to this day. Likewise, initial attempts at simulating the evolution of the disrupted star with the so-called affine method have been historically very useful. In this Chapter, we provide an overview of the numerical techniques used in the field and of their limitations, and summarize the work that has been done to simulate numerically the tidal disruption process. [ABSTRACT FROM AUTHOR]

We present optical imaging and spectroscopic studies of the Fanaroff & Riley class I (FR I) radio galaxy CTD 086 based on Hubble Space Telescope ( HST) and Sloan Digital Sky Survey ( SDSS) observations. We use isophote shape analysis to show that there is no stellar disk component within CTD 086 and further that the morphological class of the galaxy is most likely E2. Optical spectroscopy of this galaxy reveals the presence of narrow emission lines only, and thus it qualifies to be termed as a narrow-line radio galaxy (type 2 AGN). We also extract stellar kinematics from the absorption-line spectra of CTD 086 using Penalized Pixel-Fitting method and derive the black hole mass $M_{BH}$ to be equal to $(8.8\pm2.4)\times10^{7}~{\mathrm{M}\odot}$ . [ABSTRACT FROM AUTHOR]

Compared with numerous X-ray dominant active galactic nuclei (AGNs) without emission-line signatures in their optical spectra, the X-ray selected AGNs with optical emission lines are probably still in the high-accretion phase of black hole growth. This paper presents an investigation on the fraction of these X-ray detected AGNs with optical emission-line spectra in 198 galaxy groups at $z<1$ in a rest frame 0.1-2.4 keV luminosity range $41.3 < \log(L_{\mathrm{X}}/\mbox{erg}\,\mbox{s}^{-1}) < 44.1$ within the Cosmological Evolution Survey (COSMOS) field, as well as its variations with redshift and group richness. For various selection criteria of member galaxies, the numbers of galaxies and the AGNs with optical emission lines in each galaxy group are obtained. It is found that, in total 198 X-ray groups, there are 27 AGNs detected in 26 groups. AGN fraction is on average less than $4.6 (\pm1.2)\%$ for individual groups hosting at least one AGN. The corrected overall AGN fraction for whole group sample is less than $0.98 (\pm0.11) \%$ . The normalized locations of group AGNs show that 15 AGNs are found to be located in group centers, including all 6 low-luminosity group AGNs ( $L_{\mbox{ 0.5--2~keV}} < 10^{42.5} \mbox{erg}\,\mbox{s}^{-1}$ ). A week rising tendency with $z$ are found: overall AGN fraction is 0.30-0.43% for the groups at $z<0.5$ , and 0.55-0.64% at $0.5 < z < 1.0$ . For the X-ray groups at $z>0.5$ , most member AGNs are X-ray bright, optically dull, which results in a lower AGN fractions at higher redshifts. The AGN fraction in isolated fields also exhibits a rising trend with redshift, and the slope is consistent with that in groups. The environment of galaxy groups seems to make no difference in detection probability of the AGNs with emission lines. Additionally, a larger AGN fractions are found in poorer groups, which implies that the AGNs in poor groups might still be in the high-accretion phase, whereas the AGN population in rich clusters is mostly in the low-accretion, X-ray dominant phase. [ABSTRACT FROM AUTHOR]

Variability of active galactic nuclei is not well understood. One possible explanation is existence of supermassive binary black holes (SMBBH) in their centres. It is expected that major mergers are common in the Universe. It is expected that each supermassive black hole of every galaxy eventually finish as a SMBBH system in the core of newly formed galaxy. Here we model the emission line profiles of active galactic nuclei (AGN) assuming that the flux and emission line shape variations are induced by supermassive binary black hole systems (SMBBH). We assume that the accreting gas inside the circumbinary (CB) disk is photo ionized by mini accretion disk emission around each SMBBH. We calculate variations of emission line flux, shifts and shapes for different parameters of SMBBH orbits. We consider cases with different masses and inclinations for circular orbits and measure the effect to the shape of emission line profiles and flux variability. [ABSTRACT FROM AUTHOR]

We review results from cosmic X-ray surveys of active galactic nuclei (AGNs) over the past $$\approx 15$$ years that have dramatically improved our understanding of growing supermassive black holes in the distant universe. First, we discuss the utility of such surveys for AGN investigations and the capabilities of the missions making these surveys, emphasizing Chandra, XMM-Newton, and NuSTAR. Second, we briefly describe the main cosmic X-ray surveys, the essential roles of complementary multiwavelength data, and how AGNs are selected from these surveys. We then review key results from these surveys on the AGN population and its evolution ('demographics'), the physical processes operating in AGNs ('physics'), and the interactions between AGNs and their environments ('ecology'). We conclude by describing some significant unresolved questions and prospects for advancing the field. [ABSTRACT FROM AUTHOR]

BL Lac objects are active nuclei, hosted in massive elliptical galaxies, the emission of which is dominated by a relativistic jet closely aligned with the line of sight. This implies the existence of a parent population of sources with a misaligned jet that have been identified with low-power radiogalaxies. The spectrum of BL Lacs, dominated by non-thermal emission over the whole electromagnetic range, together with bright compact radio cores, high luminosities, rapid and large amplitude flux variability at all frequencies and strong polarization makes these sources an optimal laboratory for high energy astrophysics. A most distinctive characteristic of the class is the weakness or absence of spectral lines, that historically hindered the identification of their nature and ever thereafter proved to be a hurdle in the determination of their distance. In this paper, we review the main observational facts that contribute to the present basic interpretation of this class of active galaxies. We overview the history of the BL Lac objects research field and their population as it emerged from multi-wavelength surveys. The properties of the flux variability and polarization, compared with those at radio, X-ray and gamma-ray frequencies, are summarized together with the present knowledge of the host galaxies, their environments, and central black hole masses. We focus this review on the optical observations, which played a crucial role in the early phase of BL Lacs studies, and in spite of extensive radio, X-ray, and recently gamma-ray observations, could represent the future major contribution to the unveiling of the origin of these sources. In particular, they could provide a firm conclusion on the long debated issue of the cosmic evolution of this class of active galactic nuclei and on the connection between formation of supermassive black holes and relativistic jets. [ABSTRACT FROM AUTHOR]

We investigate the dependence of the Fanaroff-Riley (FR) 1/2 dichotomy of radio galaxies on their luminosities and redshifts. Because of a very strong redshift-luminosity correlation (Malmquist bias) in a flux-limited sample, any redshift-dependent effect could appear as a luminosity-related effect and vice versa. A question could then arise-do all the morphological differences seen in the two classes (FR 1 and 2 types) of sources, usually attributed to the differences in their luminosities, could as well be primarily a redshift-dependent effect? A sharp break in luminosity, seen among the two classes, could after all reflect a sharp redshift dependence due to a rather critical ambient density value at some cosmic epoch. A doubt on these lines does not seem to have been raised in past and things have never been examined with this particular aspect in mind. We want to ascertain the customary prevalent view in the literature that the systematic differences in the two broad morphology types of FR 1 and 2 radio galaxies are indeed due to the differences in their luminosities, and not due to a change in redshift. Here we investigate the dependence of FR 1/2 dichotomy of radio galaxies on luminosity and redshift by using the 3CR sample, where the FR 1/2 dichotomy was first seen, supplemented by data from an additional sample (MRC), that goes about a factor of 5 or more deeper in flux-density than the original 3CR sample. This lets us compare sources with similar luminosities but at different redshifts as well as examine sources at similar redshifts but with different luminosities, thereby allowing us a successful separation of the otherwise two intricately entangled effects. We find that the morphology type is not directly related to redshift and the break between the two types of morphologies seems to depend only upon the radio luminosity. [ABSTRACT FROM AUTHOR]

Almost all galaxies have massive central black holes in their centers with masses typically ranging from ∼10 to ∼10 M. However, the origin and evolution of these objects and their connection with the hosting galaxies are not completely understood yet. In this work we analyze the mass accretion rate of supermassive black holes (SMBH's) and the mean Eddington ratio (MER) of type 1 AGN using data from the Sloan Sky Survey. For this purpose we improve the method for constructing the subsample of SMBH, taking into account the survey flux limit and the bias of the sample. It was observed that the mean bolometric luminosity of the active black holes can be represented by a function composed by a power law in mass and a like-Schechter function in redshift. Our results also show that both the mean Eddington ratio and the mass accretion rate are proportional to this function. [ABSTRACT FROM AUTHOR]

We study the correlation between the emission from the broad-line region (BLR) and the emission in other wavelength ranges (from radio to x-ray) for the sample of 37 blazars (25 flat-spectrum radio quasars (FSRQs) and 10 BLLac blazars). Studying the relation between luminosities in various wavebands and the BLR luminosity is an effective method to examine the connection between the accretion rate and the luminosity of the jet.We used simultaneous RATAN-600measurements of blazar flux densities at six frequencies: 1.1, 2.3, 4.8, 7.7, 11.2, and 21.7 GHz. The observational data from other bands was taken from the literature. To determine the effect produced by the state of the object on the correlation, for the radio data we used the measurements obtained with the RATAN-600 in two states-namely, the maximum and minimum flux density values.We show that at some frequencies of the radio band, there exists a correlation of emission with the emission in the BLR for two types of blazars. In the FSRQ and BL Lac blazars, the correlation between the flux from the BLR and the flux in the radio band is indistinguishable in all cases, except for the case when a strong flux density variation was considered for the BL Lac-type of blazars (tens of percent). At the same time, the levels of significance for BL Lac (at certain frequencies p is worse than 0.05) in the active state indicate only the probable presence of connection. On the example of the sample, we show that the variability of emission significantly affects the level of correlation. Our results are consistent with the theoretical predictions about the close relationship of the accretion disk and the jet in blazars. [ABSTRACT FROM AUTHOR]

A fundamental role is attributed to supermassive black holes (SMBH), and the feedback they generate, in the evolution of galaxies. But theoretical models trying to reproduce the M SMBH vs. sigma relation (between the SMBH mass and stellar velocity dispersion of the galaxy bulge) make broad assumptions about the physical processes involved. These assumptions are needed due to the scarcity of observational constraints on the relevant physical processes which occur when the SMBH is being fed via mass accretion in active galactic nuclei (AGN). In search for these constraints, our group— AGN integral field spectroscopy (AGNIFS)—has been mapping the gas kinematics as well as the stellar population properties of the inner few hundred parsecs of a sample of nearby AGN hosts. In this contribution, I report on results obtained so far which show gas inflows along nuclear spirals and compact disks in the inner tens to hundreds of pc in nearby AGN hosts which seem to be the sources of fuel to the AGN. As the inflow rates are much larger than the AGN accretion rate, the excess gas must be depleted via formation of new stars in the bulge. Indeed, in many cases, we find ∼100 pc circumnuclear rings of recent star formation (ages ∼10–500 Myr) that can be interpreted as a signature of coevolution of the host galaxy and its AGN. I also report on the mapping of outflows in ionized gas, which are ubiquitous in Seyfert galaxies, and discuss mass outflow rates and powers. [ABSTRACT FROM AUTHOR]

Estimates of magnetic fields for a number of active galactic nuclei are presented. These estimates are based on the observed polarization degrees and position angles of broad H α lines and in the nearby continuum and on asymptotic analytical formulas for the Stokes parameters of the radiation emerging from a magnetized accretion disk (the Milne problem in a magnetized atmosphere). The characteristic observed feature of the wavelength dependence of the polarization degree inside the line-a minimum at the center and a fast increase of the position angle from one wing to another-can be explained by the superposition of resonance emission from two or more clouds located in the right (Keplerian velocity directed away from the observer) and left (Keplerian velocity directed toward the observer) parts of the orbit in the rotating magnetized accretion disk. The main component in our mechanism is the azimuthal magnetic field in the disk. The presence of a magnetic field perpendicular to the disk plane (which is usually weaker than the azimuthal field) results in the asymmetry of the distribution of the polarization degree and position angle inside the line. The inferred magnetic field strengths at the galactocentric distances where broad lines are emitted can be used to estimate the magnetic fields in the region of the centermost stable orbit and at the horizon of the central black hole, using the power-law dependence of the magnetic field strength corresponding to the standard model of the accretion disk. [ABSTRACT FROM AUTHOR]

We describe the technique of spectropolarimetric observations allowing for the measurements of the Stokes parameters in one of the observational modes of the SCORPIO focal reducer of the 6-m BTA telescope of the SAO RAS. The characteristics of the instrument in the spectropolarimetric mode of observations are given. We present the algorithm of observational data reduction. The capabilities of the SCORPIO spectropolarimetric mode are demonstrated on the examples of observations of various astronomical objects. [ABSTRACT FROM AUTHOR]

With the means of panoramic spectroscopy at the SAO RAS BTA telescope, we investigated the properties of stellar populations in the central regions of five early-type galaxies-the NGC524 group members. The evolution of the central regions of galaxies looks synchronized: the average age of stars in the bulges of all the five galaxies lies in the range of 3-6Gyr.Four of the five galaxies revealed synchronized bursts of star formation in the nuclei 1-2 Gyr ago. The only galaxy, in which the ages of stellar population in the nucleus and in the bulge coincide (i.e. the nuclear burst of star formation did not take place) isNGC502, the farthest from the center of the group of all the galaxies studied. [ABSTRACT FROM AUTHOR]

We present the results of radio continuum observations at frequencies ranging from ~150-5000 MHz of the misaligned double-double radio galaxy (DDRG) 3C293 (J1352+3126) using the GMRT and the VLA, and estimate the time-scale of interruption of jet activity to be less than ~0.1 Myr. [ABSTRACT FROM AUTHOR]

It has been suggested that the narrow cores of the Fe K α emission lines in Active Galactic Nuclei (AGNs) are likely produced in the torus, the inner radius of which can be measured by observing the lag time between the V and K band flux variations. In this paper, we compare the virial products of the infrared time lags, and the narrow Fe K α widths for 10 type 1 AGNs, with the black hole masses from other techniques. We found the narrow Fe K α line width is in average 2.6 times broader than expected, assuming an isotropic velocity distribution of the torus at the distance measured by the infrared lags. We propose the thick disk model of the torus may explain the observed larger line width. Another possibility is the contamination by emission from the broad line region or the outer accretion disk. Alternatively, the narrow iron line might originate from the inner most part of the obscuring torus within the sublimation radius, while the infrared emission may be from the outer cooler part. We note the correlations between the black hole masses based on this new technique and those based on other known techniques are statistically insignificant. We argue that this could be attributed to the small sample size and the very large uncertainties in the measurements of iron K line widths. The next generation of X-ray observatories could help verify the origin of the narrow iron K α line and the reliability of this new technique. [ABSTRACT FROM AUTHOR]

The orientations of the accretion disk of active galactic nuclei (AGN) and the stellar disk of its host galaxy are both determined by the angular momentum of their forming gas, but on very different physical environments and spatial scales. Here we show the evidence that the orientation of the stellar disk is correlated with the accretion disk by comparing the inclinations of the stellar disks of a large sample of Type 2 AGNs selected from Sloan Digital Sky Survey (SDSS, York et al. 2000) to a control galaxy sample. Given that the Type 2 AGN fraction is in the range of 70-90 percent for low luminosity AGNs as a priori, we find that the mean tilt between the accretion disk and stellar disk is ~ 30 degrees (Shen et al. 2010). [ABSTRACT FROM AUTHOR]

We present the results of the monitoring of a sample of 5 HBLs in the B, V and R bands during 2003–2008. All sources in our monitoring project show significant rapid optical variations, except 1ES 1959+650, however; for 1ES 1959+650, the gamma-ray variability on a timescale of ∼7 hours has been observed by Holder et al. (Astrophys. J. 583:L9, ). Using these variability timescales, we estimate the black hole masses for each source with the Kerr black hole theory. [ABSTRACT FROM AUTHOR]

We explore the role of star clusters in the nuclear regions of galaxies through their connection with active galactic nuclei (AGN). Nuclear star clusters (NCs) are conspicuous in the centers of most nearby galaxies, all along the Hubble sequence. These clusters are probably the faint-end distribution of the central supermassive black holes (SMBHs) in massive bulges. On the other hand, star formation is known to be ongoing in the majority of Seyfert nuclei and in many low-luminosity active galactic nuclei (LLAGN). We study two samples of AGN galaxies (75 Seyferts and 26 LLAGN) in the near-ultraviolet with the Hubble Space Telescope’s Advanced Camera for Surveys. We aim to better understand the connection between the growing of the SMBH and the build-up of the bulge, and we also intend to make statistical progress and determine the properties of the population of NCs coexisting with growing SMBHs. [ABSTRACT FROM AUTHOR]

The power spectral densities (PSDs) for a sample of active galactic nuclei (AGNs) are analyzed in both the frequency domain and the time domain. We find for each object that for broadband noise a character timescale-bifurcation timescale of Fourier and time-domain PSD exists in the 103–106 s range, below which the time-domain power spectrum is systematically higher than the corresponding Fourier spectrum. The relationship between bifurcation timescale, AGN mass and luminosity is studied. Compared with the fact that similar phenomena have been found for Galactic black hole candidates (GBHs) with bifurcation timescale ∼0.1 s but not for accreting neutron stars, our finding indicates that AGNs and GBHs have common intrinsic nature in rapid X-ray variability with a character time parameter scaled with their masses [ABSTRACT FROM AUTHOR]

Nuclear stellar clusters are a common phenomenon in spirals and in starburst galaxies, and they may be a natural consequence of the star formation processes in the central regions of galaxies. HST UV imaging of a few Seyfert 2 galaxies have resolved nuclear starbursts in Seyfert 2 revealing stellar clusters as the main components of the extended emission. However, we do not know whether stellar clusters are always associated with all types of nuclear activity. We present HST NUV and optical images to study the role that stellar clusters play in different types of AGNs. Also with these images, we study the circumnuclear dust morphology as a probe of the circumnuclear environment of AGNs. [ABSTRACT FROM AUTHOR]

Compact relativistic jets in active galactic nuclei offer an effective tool for investigating the physics of nuclear regions in galaxies. The emission properties, dynamics, and evolution of jets in AGN are closely connected to the characteristics of the central supermassive black hole, accretion disk and broad-line region in active galaxies. Recent results from studies of the nuclear regions in several active galaxies with prominent outflows are reviewed in this contribution. [ABSTRACT FROM AUTHOR]

Fifty-three spectrograms in the optical region (3700–7300 Å) with the spectral resolution ∼8 Å have been obtained for the Seyfert nucleus of the galaxy NGC 3227 with the 6-m telescope on January 1977, while the nucleus was in the historically important epoch of its extreme maximum brightness. Width of the slit was 1″, length of the box during the spectra measurements was 1.5″. Data obtained by us and those compiled from literature showed that profiles of the Balmer lines Hα, Hβ and Hγ are different, evidencing that the gas emitting these lines is highly self-absorbed. It was shown that narrow components of the profiles revealed by Rubin and Ford kept their positions (radial velocities) over 25 years. The components showed intensity variations compared to the central one from minimum to maximum of the nucleus brightness. The same variations were observed by us earlier in the emission line profiles of the NGC 7469 nucleus spectrum. Narrow profile components can reflect long-lived flows or jets in the broad line region (BLR). Obtained facts evidenced that long-lived gas streams and flows causing narrow components of broad line profiles presented not only when BLR of accretion disc is strong but when BLR of accretion disc declined. Blue bump at radial velocity of −5000 km/s in Hγ profile was revealed in spectra of high states of the nucleus, which disappeared in low state. One of the interpretations of this event can be in the framework of a model of one-sided or two-sided gas ejection during the high state of the nucleus, positive radial velocities of which being screened out by a circumnuclear disk. [ABSTRACT FROM AUTHOR]

We present preliminary results of recent spatially resolved mid infrared (Mid-IR) spectroscopy with the MICHELLE instrument on UKIRT of NGC 7469, observed as part of a larger programme to study a sample of starburst and AGN in the Mid-IR. [ABSTRACT FROM AUTHOR]

We report three new or updated techniques for probing the parameters of active galaxies based on the masses of their central black holes (MBH). First, we derived a near-IR analog of the bulge luminosity versus MBH relationship. The low scatter makes it a promising new tool to study the black hole demographics. Next, we present relations between MBH and the 10μm and 2–10 keV nuclear luminosity. They may help to study the MBH evolution over wide redshift ranges. Finally, we measured MBH in quasars from z ∼ 3.4 to z ∼ 0.3 to search directly for MBH growth. Surprisingly, we found no evidence for growth implying that the majority of quasar host galaxies have undergone their last major merger at z &gsim; 3. [ABSTRACT FROM AUTHOR]

Nearby luminous galaxies serve as templates to gain insight into the building blocks of the most luminous galaxies in the distant universe. In this conference I have shown that the most luminous galaxies in the Local Universe are mergers of evolved gas-rich galaxies that radiate the bulk of their energy in the mid and far-infrared. The most important implication for future studies on the formation of galaxies at early cosmological timescales is that they will appear as luminous sub-millimeter sources. [ABSTRACT FROM AUTHOR]

The evolution of galaxies has been driven by galaxy-galaxy collisions on all cosmological time-scales, from the primordial universe to the present. In this context, nearby mergers serve as local analogs to gain insight into the physical processes that lead to the formation and trans-formation of galaxies in the more distant universe. Here I review multiwave length observations -with particular emphasis on recent results obtained with ISO- of mergers of massive galaxies driving the formation of: 1) luminous infrared galaxies, 2) elliptical galaxy cores, 3) luminous dust-enshrouded extra nuclear starbursts, 4) symbiotic galaxies, and 5) tidal dwarf galaxies. The most important implication for future studies on the formation of galaxies at early cosmological timescales is that the most luminous galaxies in the local universe (z≤0.1) emit the bulk of their energy in the mid and far-infrared, and therefore, their analogs in the more distant universe would be invisible in the ultraviolet and optical wavelength rest-frames. [ABSTRACT FROM AUTHOR]

Processes of collimation of electrically charged particles near a rotating black hole are discussed. It is assumed that the black hole is immersed in a weak magnetic field aligned with rotation axis. This situation is relevant for understanding pre-collimation of astrophysical jets. The magnetic field affects the motion of material and restricts the validity of various scenarios which use the test-particle (cold plasma) approximation. A simplified criterion to estimate the relevance of this approximation is discussed in connection with the mechanism of the dissipative collimation, as proposed by de Felice and Curir. [ABSTRACT FROM AUTHOR]

Mergers are fundamental to the standard paradigm of galaxy evolution, and provide a natural formation mechanism for supermassive black hole binaries. The formation process of such a binary can have a direct impact on the rate at which stars are tidally disrupted by one or the other black hole, and the luminous signature of the tidal disruption itself can have distinct imprints of a binary companion. In this article we review our current understanding of the influence of black hole binarity on the properties of tidal disruption events. We discuss the rates of tidal disruption by supermassive black hole binaries, the impact of a second black hole on the fallback of debris and the formation of an accretion flow, and the prospects for detection of tidal disruption events by supermassive black hole binaries. [ABSTRACT FROM AUTHOR]