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Developing the final summative assessment of a course at the start of curriculum development is an implementation of "backward design," whereby learning objectives are identified first and the curriculum is engineered end-to-beginning to achieve them. We trained in backward design through the Professional Development Program (PDP) and adapted PDP assessment ideas for evaluation of curriculum designs and teaching efficacy. A pre-/post-quiz is an assessment administered the first and last day of a course; a learner's scores are used to measure normalized gain: the ratio of what a student learned during a course relative to what they knew entering it. The intentional process of developing a pre-/post-quiz for every course focuses the educator on the essential understanding desired of the learners exiting the course. The normalized-gain statistics for the course can then be used to evaluate the course's efficacy, and improvements to the curriculum can be monitored by tracking the normalized gains over time, using the same pre-/post-quiz. Moreover, an individual instructor may self-evaluate their teaching efficacy by tracking normalized gains from all courses over time. Here we discuss applying the practice of backward curriculum design starting with a custom pre-/post-quiz and utilizing it for immediate and longitudinal evaluation, focusing primarily on designing an entire undergraduate science course., Comment: 17 pages, 2 figures; in S. Seagroves, A. Barnes, A.J. Metevier, J. Porter, & L. Hunter (Eds.), Leaders in effective and inclusive STEM: Twenty years of the Institute for Scientist & Engineer Educators. UC Santa Cruz: Institute for Scientist & Engineer Educators; see https://escholarship.org/uc/isee_pdp20yr

We present an analysis of the spatial distribution of star formation in a sample of 60 visually identified galaxy merger candidates at z>1. Our sample, drawn from the 3D-HST survey, is flux-limited and was selected to have high star formation rates based on fits of their broad-band, low spatial resolution spectral energy distributions. It includes plausible pre-merger (close pairs) and post-merger (single objects with tidal features) systems, with total stellar masses and star formation rates derived from multi-wavelength photometry. Here we use near-infrared slitless spectra from 3D-HST which produce Halpha or [OIII] emission line maps as proxies for star-formation maps. This provides a first comprehensive high-resolution, empirical picture of where star formation occurred in galaxy mergers at the epoch of peak cosmic star formation rate. We find that detectable star formation can occur in one or both galaxy centres, or in tidal tails. The most common case (58%) is that star formation is largely concentrated in a single, compact region, coincident with the centre of (one of) the merger components. No correlations between star formation morphology and redshift, total stellar mass, or star formation rate are found. A restricted set of hydrodynamical merger simulations between similarly massive and gas-rich objects implies that star formation should be detectable in both merger components, when the gas fractions of the individual components are the same. This suggests that z~1.5 mergers typically occur between galaxies whose gas fractions, masses, and/or star formation rates are distinctly different from one another., Comment: Accepted for publication in MNRAS, 16 pages, 10 figures

We measured and modeled the spectral energy distributions (SEDs) in 28 bands from the ultraviolet to the far-infrared (FIR) for 31 interacting galaxies in 14 systems. The sample is drawn from the Spitzer Interacting Galaxy Survey, which probes a range of galaxy interaction parameters at multiple wavelengths with an emphasis on the infrared bands. The subset presented in this paper consists of all galaxies for which FIR Herschel SPIRE observations are publicly available. Our SEDs combine the Herschel photometry with multi-wavelength data from Spitzer, GALEX, Swift UVOT, and 2MASS. While the shapes of the SEDs are broadly similar across our sample, strongly interacting galaxies typically have more mid-infrared emission relative to their near-infrared and FIR emission than weakly or moderately interacting galaxies. We modeled the full SEDs to derive host galaxy star formation rates (SFR), specific star formation rates (sSFR), stellar masses, dust temperatures, dust luminosities, and dust masses. We find increases in the dust luminosity and mass, SFR, and cold (15-25 K) dust temperature as the interaction progresses from moderately to strongly interacting and between non- interacting and strongly interacting galaxies. We also find increases in the SFR between weakly and strongly interacting galaxies. In contrast, the sSFR remains unchanged across all the interaction stages. The SFR derived from the SED modeling agrees well with rates estimated by proportionality relations that depend on infrared emission. (abridged), Comment: 37 emulateapj pages, 11 tables, 25 figures, re-submitted to ApJ after revision per referee report

We analyze the link between active galactic nuclei (AGN) and mid-infrared flux using dust radiative transfer calculations of starbursts realized in hydrodynamical simulations. Focusing on the effects of galaxy dust, we evaluate diagnostics commonly used to disentangle AGN and star formation in ultraluminous infrared galaxies (ULIRGs). We examine these quantities as a function of time, viewing angle, dust model, AGN spectrum, and AGN strength in merger simulations representing two possible extremes of the ULIRG population: one is a typical gas-rich merger at z ~ 0, and the other is characteristic of extremely obscured starbursts at z ~ 2 to 4. This highly obscured burst begins star-formation-dominated with significant PAH emission, and ends with a ~10^9 yr period of red near-IR colors. At coalescence, when the AGN is most luminous, dust obscures the near-infrared AGN signature, reduces the relative emission from polycyclic aromatic hydrocarbons (PAHs), and enhances the 9.7 micron absorption by silicate grains. Although generally consistent with previous interpretations, our results imply none of these indicators can unambiguously estimate the AGN luminosity fraction in all cases. Motivated by the simulations, we show that a combination of the extinction feature at 9.7 micron, the PAH strength, and a near-infrared slope can simultaneously constrain the AGN fraction and dust grain distribution for a wide range of obscuration. We find that this indicator, accessible to the James Webb Space Telescope, may estimate the AGN power as tightly as the hard X-ray flux alone, thereby providing a valuable future cross-check and constraint for large samples of distant ULIRGs., Comment: 21 pages, 13 figures, ApJ accepted version

High-redshift submillimetre galaxies (SMGs) are some of the most rapidly star-forming galaxies in the Universe. Historically, galaxy formation models have had difficulty explaining the observed number counts of SMGs. We combine a semi-empirical model with 3-D hydrodynamical simulations and 3-D dust radiative transfer to predict the number counts of unlensed SMGs. Because the stellar mass functions, gas and dust masses, and sizes of our galaxies are constrained to match observations, we can isolate uncertainties related to the dynamical evolution of galaxy mergers and the dust radiative transfer. The number counts and redshift distributions predicted by our model agree well with observations. Isolated disc galaxies dominate the faint (S_1.1 < ~1 mJy, or S_850 < ~2 mJy) population. The brighter sources are a mix of merger-induced starbursts and galaxy-pair SMGs; the latter subpopulation accounts for ~30 - 50 per cent of all SMGs at all S_1.1 > ~0.5 mJy (S_850 > ~1 mJy). The mean redshifts are ~3.0 - 3.5, depending on the flux cut, and the brightest sources tend to be at higher redshifts. Because the galaxy-pair SMGs will be resolved into multiple fainter sources by ALMA, the bright ALMA counts should be as much as 2 times less than those observed using single-dish telescopes. The agreement between our model, which uses a Kroupa IMF, and observations suggests that the IMF in high-redshifts starbursts need not be top-heavy; if the IMF were top-heavy, our model would over-predict the number counts. We conclude that the difficulty some models have reproducing the observed SMG counts is likely indicative of more general problems -- such as an under-prediction of the abundance of massive galaxies or a star formation rate--stellar mass relation normalisation lower than that observed -- rather than a problem specific to the SMG population., Comment: Accepted for publication in MNRAS. Insignificant changes from the previous version

The definition of homology for single-component galaxies is clear, but for multi-component (luminous and dark matter) galaxies there is some ambiguity. We attempt to clarify the situation by carefully separating the different concepts of homology that have been used to date. We argue that the most useful definition is that a set of galaxies is homologous if they are the same in all respects up to a set of three dimensional scaling constants which may differ from one galaxy to the next. Noting that we are free to choose the dimensional constants, we find that a set of hydrodynamic simulated galaxy merger remnants is significantly closer to homologous when the dimensional length constant is taken to be the radius containing equal amounts of dark and baryonic matter rather than the usual observationally motivated choice of the baryonic half-mass radius. Once the correct dimensional scaling constants are used, the stellar velocity dispersion anisotropy is essentially the sole source of the variation in the kinematic structure of these simulated merger remnants. In order to facilitate the use of these scaling constants to analyse observed galaxies, we calculated the relationship between our preferred dimensional scaling constants and the typical observationally accessible quantities., Comment: 15 pages, 9 figures, 3 tables. Accepted to MNRAS

In recent work (arXiv:1101.0002) we have suggested that the high-redshift (z ~ 2-4) bright submillimetre galaxy (SMG) population is heterogeneous, with major mergers contributing both at early stages, where quiescently star-forming discs are blended into one submm source (`galaxy-pair SMGs'), and late stages, where mutual tidal torques drive gas inflows and cause strong starbursts. Here we combine hydrodynamic simulations of major mergers with 3-D dust radiative transfer calculations to determine observational diagnostics that can distinguish between quiescently star-forming SMGs and starburst SMGs via integrated data alone. We fit the far-IR SEDs of the simulated galaxies with the optically thin single-temperature modified blackbody, the full form of the single-temperature modified blackbody, and a power-law temperature-distribution model. The effective dust temperature, T_dust, and power-law index of the dust emissivity in the far-IR, \beta, derived can significantly depend on the fitting form used, and the intrinsic \beta\ of the dust is not recovered. However, for all forms used here, there is a T_dust above which almost all simulated galaxies are starbursts, so a T_dust cut is very effective at selecting starbursts. Simulated merger-induced starbursts also have higher L_IR/M_gas and L_IR/L_FUV than quiescently star-forming galaxies and lie above the star formation rate-stellar mass relation. These diagnostics can be used to test our claim that the SMG population is heterogeneous and to observationally determine what star formation mode dominates a given galaxy population. We comment on applicability of these diagnostics to ULIRGs that would not be selected as SMGs. These `hot-dust ULIRGs' are typically starburst galaxies lower in mass than SMGs, but they can also simply be SMGs observed from a different viewing angle., Comment: 21 pages, 11 figures. Accepted for publication in MNRAS. Minor changes to text but otherwise identical to v1

The evolution of galaxies is greatly influenced by their interactions. As part of a program to study interacting galaxies, we have measured and modeled the spectral energy distri- butions (SEDs) from the ultraviolet (UV) to the far-infrared (FIR). We describe the constraints imposed on star formation histories by these SEDs, and the variations therein seen across the interaction sequence, and we compare the results of different star formation rate prescriptions applied to the data. The sample itself is based on the Spitzer Interacting Galaxy Survey (SIGS) of 111 galaxies in 50 systems, a project designed to probe a range of galaxy interaction parameters in the infrared. Our SEDs combine the Spitzer results with multiwavelength data from other missions, in particular GALEX and Herschel. The subset presented here is the sample for which FIR Herschel observations are currently publicly available., Comment: 5 pages, 4 figures, to appear in the proceedings of "The Spectral Energy Distribution of Galaxies", Preston England, September 2011

Calculating the galaxy merger rate requires both a census of galaxies identified as merger candidates, and a cosmologically-averaged `observability' timescale T_obs(z) for identifying galaxy mergers. While many have counted galaxy mergers using a variety of techniques, T_obs(z) for these techniques have been poorly constrained. We address this problem by calibrating three merger rate estimators with a suite of hydrodynamic merger simulations and three galaxy formation models. We estimate T_obs(z) for (1) close galaxy pairs with a range of projected separations, (2) the morphology indicator G-M20, and (3) the morphology indicator asymmetry A. Then we apply these timescales to the observed merger fractions at z < 1.5 from the recent literature. When our physically-motivated timescales are adopted, the observed galaxy merger rates become largely consistent. The remaining differences between the galaxy merger rates are explained by the differences in the range of mass-ratio measured by different techniques and differing parent galaxy selection. The major merger rate per unit co-moving volume for samples selected with constant number density evolves much more strongly with redshift (~ (1+z)^(+3.0 \pm 1.1)) than samples selected with constant stellar mass or passively evolving luminosity (~ (1+z)^(+0.1 \pm 0.4)). We calculate the minor merger rate (1:4 < M_{sat}/M_{primary} <~ 1:10) by subtracting the major merger rate from close pairs from the `total' merger rate determined by G-M20. The implied minor merger rate is ~3 times the major merger rate at z ~ 0.7, and shows little evolution with redshift., Comment: accepted by ApJ; 25 pages; 13 figures; 4 tables

Models of poststarburst (or "K+A") galaxies are constructed by combining fully three-dimensional hydrodynamic simulations of galaxy mergers with radiative transfer calculations of dust attenuation. Spectral line catalogs are generated automatically from moderate-resolution optical spectra calculated as a function of merger progress in each of a large suite of simulations. The mass, gas fraction, orbital parameters, and mass ratio of the merging galaxies are varied systematically, showing that the lifetime and properties of the K+A phase are strong functions of merger scenario. K+A durations are generally less than ~0.1-0.3 Gyr, significantly shorter than the commonly assumed 1 Gyr, which is obtained only in rare cases, owing to a wide variation in star formation histories resulting from different orbital and progenitor configurations. Combined with empirical merger rates, the model lifetimes predict rapidly-rising K+A fractions as a function of redshift that are consistent with results of large spectroscopic surveys, resolving tension between the observed K+A abundance and that predicted when one assumes the K+A duration is the lifetime of A stars (~1 Gyr). The effects of dust attenuation, viewing angle, and aperture bias on our models are analyzed. In some cases, the K+A features are longer-lived and more pronounced when AGN feedback removes dust from the center, uncovering the young stars formed during the burst. In this picture, the K+A phase begins during or shortly after the bright starburst/AGN phase in violent mergers, and thus offers a unique opportunity to study the effects of quasar and star formation feedback on the gas reservoir and evolution of the remnant. Analytic fitting formulae are provided for the estimates of K+A incidence as a function of merger scenario., Comment: 26 pages, 13 figures; ApJ; minor changes to reflect accepted version

We perform 3-D dust radiative transfer (RT) calculations on hydrodynamic simulations of isolated and merging disk galaxies in order to quantitatively study the dependence of observed-frame submillimeter (submm) flux density on galaxy properties. We find that submm flux density and star formation rate (SFR) are related in dramatically different ways for quiescently star-forming galaxies and starbursts. Because the stars formed in the merger-induced starburst do not dominate the bolometric luminosity and the rapid drop in dust mass and more compact geometry cause a sharp increase in dust temperature during the burst, starbursts are very inefficient at boosting submm flux density (e.g., a $\ga16$x boost in SFR yields a $\la 2$x boost in submm flux density). Moreover, the ratio of submm flux density to SFR differs significantly between the two modes; thus one cannot assume that the galaxies with highest submm flux density are necessarily those with the highest bolometric luminosity or SFR. These results have important consequences for the bright submillimeter-selected galaxy (SMG) population. Among them are: 1. The SMG population is heterogeneous. In addition to merger-driven starbursts, there is a subpopulation of galaxy pairs, where two disks undergoing a major merger but not yet strongly interacting are blended into one submm source because of the large ($\ga 15$", or $\sim 130$ kpc at $z = 2$) beam of single-dish submm telescopes. 2. SMGs must be very massive ($M_{\star} \ga 6 \times 10^{10} \msun$). 3. The infall phase makes the SMG duty cycle a factor of a few greater than what is expected for a merger-driven starburst. (Abridged.), Comment: 15 pages, 3 figures. Accepted for publication in ApJ. Minor changes from v1

A sample of very high resolution cosmological disk galaxy simulations is used to investigate the evolution of galaxy disk sizes back to redshift 1 within the Lambda CDM cosmology. Artificial images in the rest frame B band are generated, allowing for a measurement of disk scale lengths using surface brightness profiles as observations would, and avoiding any assumption that light must follow mass as previous models have assumed. We demonstrate that these simulated disks are an excellent match to the observed magnitude - size relation for both local disks, and for disks at z=1 in the magnitude/mass range of overlap. We disentangle the evolution seen in the population as a whole from the evolution of individual disk galaxies. In agreement with observations, our simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness dimming since z=1. We find evidence that evolution in the magnitude - size plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more evolution in size than luminosity, while dwarf galaxies tend to evolve potentially more in luminosity. The disks grow in such a way as to stay on roughly the same stellar mass - size relation with time. Finally, due to an evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or size) at z=1 will reside in a more massive halo and have a higher SFR, and thus a higher luminosity, than a counterpart of the same stellar mass at z=0., Comment: Version resubmitted to ApJ, after referee's comments

Theoretical models have had difficulty matching the observed number density of sub-millimeter galaxies (SMGs), causing some authors (e.g., Baugh et al. 2005) to suggest that SMGs provide evidence for a top-heavy initial mass function (IMF). To test this claim, we have, for the first time, combined high-resolution 3-D hydrodynamic simulations of isolated and merging massive, gas-rich galaxies, radiative transfer, and a semi-empirical merger rate model to predict the number density of SMGs. Our model can reproduce the observed SMG number density even when using a standard (Kroupa) IMF. The agreement is due to a combination of relatively long sub-mm duty cycles for mergers (a few times 10^8 years for our most massive models), which owe to our combination of high-resolution 3-D hydrodynamic simulations and dust radiative transfer; sufficient number densities of massive, gas-rich mergers; and the decrease in sub-mm counts observed by recent deep/wide surveys (e.g., Austermann et al. 2010) relative to previous surveys. Our results suggest that the observed SMG number counts do not provide evidence for a top-heavy IMF at high redshift., Comment: Minor typo in the abstract corrected

We present a detailed analysis of the structure and resolved stellar populations of simulated merger remnants, and compare them to observations of compact quiescent galaxies at z ~ 2. We find that major merging is a viable mechanism to produce systems of ~ 10^11 Msun and ~ 1 kpc size, provided the gas fraction at the time of final coalescence is high (~ 40%), and provided that the progenitors are compact star-forming galaxies, as expected at high redshift. Their integrated spectral energy distributions and velocity dispersions are in good agreement with the observations, and their position in the (v_{maj}/sigma, ellipticity) diagram traces the upper envelope of the distribution of lower redshift early-type galaxies. The simulated merger remnants show time- and sightline-dependent M/L ratio gradients that result from a superposition of radially dependent stellar age, stellar metallicity, and extinction. The median ratio of effective radius in rest-frame V-band light to that in mass surface density is ~ 2 during the quiescent remnant phase. This is typically expressed by a negative color gradient (i.e., red core), which we expect to correlate with the integrated color of the system. Finally, the simulations differ from the observations in their surface brightness profile shape. The simulated remnants are typically best fit by high (n >> 4) Sersic indices, whereas observed quiescent galaxies at z ~ 2 tend to be less cuspy (median n ~ 2.3). Limiting early star formation in the progenitors may be required to prevent the simulated merger remnants from having extended wings., Comment: Accepted for publication in The Astrophysical Journal, 21 pages, 17 figures

We use cosmological hydrodynamical simulations of the formation of Milky Way-mass galaxies to study the relative importance of the main stellar components, i.e., discs, bulges, and bars, at redshift zero. The main aim of this work is to understand if estimates of the structural parameters of these components determined from kinematics (as is usually done in simulations) agree well with those obtained using a photometric bulge/disc/bar decomposition (as done in observations). To perform such a comparison, we have produced synthetic observations of the simulation outputs with the Monte-Carlo radiative transfer code SUNRISE and used the BUDDA code to make 2D photometric decompositions of the resulting images (in the i and g bands). We find that the kinematic disc-to-total ratio (D/T) estimates are systematically and significantly lower than the photometric ones. While the maximum D/T ratios obtained with the former method are of the order of 0.2, they are typically >0.4, and can be as high as 0.7, according to the latter. The photometric decomposition shows that many of the simulated galaxies have bars, with Bar/T ratios in the range 0.2-0.4, and that bulges have in all cases low Sersic indices, resembling observed pseudo-bulges instead of classical ones. Simulated discs, bulges and bars generally have similar (g-i) colours, which are in the blue tail of the distribution of observed colours. This is not due to the presence of young stars, but rather to low metallicities and poor gas content in the simulated galaxies, which makes dust extinction low. Photometric decompositions thus match the component ratios usually quoted for spiral galaxies better than kinematic decompositions, but the shift is insufficient to make the simulations consistent with observed late-type systems., Comment: MNRAS Letters accepted, revised version, conclusions unchanged

Gas-rich galaxy mergers are more easily identified by their disturbed morphologies than mergers with less gas. Because the typical gas fraction of galaxy mergers is expected to increase with redshift, the under-counting of low gas-fraction mergers may bias morphological estimates of the evolution of galaxy merger rate. To understand the magnitude of this bias, we explore the effect of gas fraction on the morphologies of a series of simulated disc galaxy mergers. With the resulting g-band images, we determine how the time-scale for identifying major and minor galaxy mergers via close projected pairs and quantitative morphology (the Gini coefficient G, the second-order moment of the brightest 20% of the light M20, and asymmetry A) depends on baryonic gas fraction f(gas). Strong asymmetries last significantly longer in high gas-fraction mergers of all mass ratios, with time-scales ranging from >= 300 Myr for f(gas) ~ 20% to >= 1 Gyr for f(gas) ~ 50%. Therefore the strong evolution with redshift observed in the fraction of asymmetric galaxies may reflect evolution in the gas properties of galaxies rather than the global galaxy merger rate. On the other hand, the time-scale for identifying a galaxy merger via G-M20 is weakly dependent on gas-fraction (~ 200-400 Myr), consistent with the weak evolution observed for G-M20 mergers., Comment: 15 pages; resubmitted to MNRAS

The majority of galaxy mergers are expected to be minor mergers. The observational signatures of minor mergers are not well understood, thus there exist few constraints on the minor merger rate. This paper seeks to address this gap in our understanding by determining if and when minor mergers exhibit disturbed morphologies and how they differ from the morphology of major mergers. We simulate a series of unequal-mass moderate gas-fraction disc galaxy mergers. With the resulting g-band images, we determine how the time-scale for identifying galaxy mergers via projected separation and quantitative morphology (the Gini coefficient G, asymmetry A, and the second-order moment of the brightest 20% of the light M20) depends on the merger mass ratio, relative orientations and orbital parameters. We find that G-M20 is as sensitive to 9:1 baryonic mass ratio mergers as 1:1 mergers, with observability time-scales ~ 0.2-0.4 Gyr. In contrast, asymmetry finds mergers with baryonic mass ratios between 4:1 and 1:1 (assuming local disc galaxy gas-fractions). Asymmetry time-scales for moderate gas-fraction major disc mergers are ~ 0.2-0.4 Gyr, and less than 0.06 Gyr for moderate gas-fraction minor mergers. The relative orientations and orbits have little effect on the time-scales for morphological disturbances. Observational studies of close pairs often select major mergers by choosing paired galaxies with similar luminosities and/or stellar masses. Therefore, the various ways of finding galaxy mergers (G-M20, A, close pairs) are sensitive to galaxy mergers of different mass ratios. By comparing the frequency of mergers selected by different techniques, one may place empirical constraints on the major and minor galaxy merger rates., Comment: 16 pages; resubmitted to MNRAS

For almost two decades the properties of "dwarf" galaxies have challenged the Cold Dark Matter (CDM) paradigm of galaxy formation. Most observed dwarf galaxies consists of a rotating stellar disc embedded in a massive DM halo with a near constant-density core. Yet, models based on the CDM scenario invariably form galaxies with dense spheroidal stellar "bulges" and steep central DM profiles, as low angular momentum baryons and DM sink to the center of galaxies through accretion and repeated mergers. Processes that decrease the central density of CDM halos have been identified, but have not yet reconciled theory with observations of present day dwarfs. This failure is potentially catastrophic for the CDM model, possibly requiring a different DM particle candidate. This Letter presents new hydrodynamical simulations in a Lambda$CDM framework where analogues of dwarf galaxies, bulgeless and with a shallow central DM profile, are formed. This is achieved by resolving the inhomogeneous interstellar medium, resulting in strong outflows from supernovae explosions which remove low angular momentum gas. This inhibits the formation of bulges and decreases the dark-matter density to less than half within the central kiloparsec. Realistic dwarf galaxies are thus shown to be a natural outcome of galaxy formation in the CDM scenario., Comment: To appear on Nature. Includes Supplemental Information. A movie can be downloaded at http://www.astro.washington.edu/users/fabio/movies/dwarfCDM2.mov

We present a physical model for the origin of z~2 Dust-Obscured Galaxies (DOGs), a class of high-redshift ULIRGs selected at 24 micron which are particularly optically faint (24/R>1000). By combining N-body/SPH simulations of high redshift galaxy evolution with 3D polychromatic dust radiative transfer models, we find that luminous DOGs (with F24 > 0.3 mJy at z~2 are well-modeled as extreme gas-rich mergers in massive (~5x10^12-10^13 Msun) halos, with elevated star formation rates (~500-1000 Msun/yr) and/or significant AGN growth (Mdot > 0.5 Msun/yr), whereas less luminous DOGs are more diverse in nature. At final coalescence, merger-driven DOGs transition from being starburst dominated to AGN dominated, evolving from a "bump" to a power-law shaped mid-IR (IRAC) spectral energy distribution (SED). After the DOG phase, the galaxy settles back to exhibiting a "bump" SED with bluer colors and lower star formation rates. While canonically power-law galaxies are associated with being AGN-dominated, we find that the power-law mid-IR SED can owe both to direct AGN contribution, as well as to a heavily dust obscured stellar bump at times that the galaxy is starburst dominated. Thus power-law galaxies can be either starburst or AGN dominated. Less luminous DOGs can be well-represented either by mergers, or by massive ($M_{\rm baryon} ~5x10^11 Msun) secularly evolving gas-rich disc galaxies (with SFR > 50 Msun/yr). By utilising similar models as those employed in the SMG formation study of Narayanan et al. (2010), we investigate the connection between DOGs and SMGs. We find that the most heavily star-forming merger driven DOGs can be selected as Submillimetre Galaxies (SMGs), while both merger-driven and secularly evolving DOGs typically satisfy the BzK selection criteria., Comment: Accepted by MNRAS; major changes include better description of dependency on ISM specification and updated models allowing dust to evolve with metallicity.

When calculating the infrared spectral energy distributions (SEDs) of galaxies in radiation-transfer models, the calculation of dust grain temperatures is generally the most time-consuming part of the calculation. Because of its highly parallel nature, this calculation is perfectly suited for massively parallel general-purpose Graphics Processing Units (GPUs). This paper presents an implementation of the calculation of dust grain equilibrium temperatures on GPUs in the Monte-Carlo radiation transfer code Sunrise, using the CUDA API. The GPU can perform this calculation 69 times faster than the 8 CPU cores, showing great potential for accelerating calculations of galaxy SEDs., Comment: 7 pages, 2 figures, accepted to New Astronomy. Minor updates to text and performance based on feedback from referee

An updated version of the dust radiation transfer code Sunrise, including models for star-forming regions and a self-consistent calculation of the spatially dependent dust and PAH emission, is presented. Given a hydrodynamic simulation of a galaxy, this model can calculate a realistic 2-dimensional ultraviolet--submillimeter spectral energy distribution of the galaxy, including emission lines from HII regions, from any viewpoint. To model the emission from star-forming regions, the MAPPINGSIII photoionization code is used. The high wavelength resolution (~ 1000 wavelengths) is made possible by the polychromatic Monte-Carlo algorithm employed by Sunrise. From the 2-D spectral energy distributions, images in any filter bands or integrated galaxy SEDs can be created. Using a suite of hydrodynamic simulations of disc galaxies, the output broad-band images and spectral energy distributions are compared with observed galaxies from the multiwavelength SINGS and SLUGS galaxy surveys. Overall, the output spectral energy distributions show a good match with observed galaxies in colours ranging from GALEX far-UV to SCUBA submillimeter wavelengths. The only possible exception is the 160 micron/850 micron colour, which the simulations underestimate by a factor "of order 5" compared to the SINGS sample. However, the simulations here agree with the SLUGS galaxies, which consistently have significantly larger amounts of cold dust than the SINGS galaxies. The Sunrise model can be used to generate simulated observations of arbitrary hydrodynamic galaxy simulations. In this way, predictions of galaxy formation theories can be directly tested against observations of galaxies., Comment: 32 pages, 22 figures, submitted to MNRAS

We describe a model for the formation of \zsim 2 Submillimeter Galaxies (SMGs) which simultaneously accounts for both average and bright SMGs while providing a reasonable match to their mean observed spectral energy distributions (SEDs). By coupling hydrodynamic simulations of galaxy mergers with the high resolution 3D polychromatic radiative transfer code Sunrise, we find that a mass sequence of merger models which use observational constraints as physical input naturally yield objects which exhibit black hole, bulge, and H2 gas masses similar to those observed in SMGs. The dominant drivers behind the 850 micron flux are the masses of the merging galaxies and the stellar birthcloud covering fraction. The most luminous (S850 ~ 15 mJy) sources are recovered by ~10^13 Msun 1:1 major mergers with a birthcloud covering fraction close to unity, whereas more average SMGs ~5-7 mJy) may be formed in lower mass halos ~5x10^12 Msun. These models demonstrate the need for high spatial resolution hydrodynamic and radiative transfer simulations in matching both the most luminous sources as well as the full SEDs of SMGs. While these models suggest a natural formation mechanism for SMGs, they do not attempt to match cosmological statistics of galaxy populations; future efforts along this line will help ascertain the robustness of these models., Comment: MNRAS Accepted; Revised version includes expanded discussion of simulated radio properties of SMGs

Astronomical software is now a fact of daily life for all hands-on members of our community. Purpose-built software for data reduction and modeling tasks becomes ever more critical as we handle larger amounts of data and simulations. However, the writing of astronomical software is unglamorous, the rewards are not always clear, and there are structural disincentives to releasing software publicly and to embedding it in the scientific literature, which can lead to significant duplication of effort and an incomplete scientific record. We identify some of these structural disincentives and suggest a variety of approaches to address them, with the goals of raising the quality of astronomical software, improving the lot of scientist-authors, and providing benefits to the entire community, analogous to the benefits provided by open access to large survey and simulation datasets. Our aim is to open a conversation on how to move forward. We advocate that: (1) the astronomical community consider software as an integral and fundable part of facility construction and science programs; (2) that software release be considered as integral to the open and reproducible scientific process as are publication and data release; (3) that we adopt technologies and repositories for releasing and collaboration on software that have worked for open-source software; (4) that we seek structural incentives to make the release of software and related publications easier for scientist-authors; (5) that we consider new ways of funding the development of grass-roots software; (6) and that we rethink our values to acknowledge that astronomical software development is not just a technical endeavor, but a fundamental part of our scientific practice., Comment: State of the Profession white paper submitted to the Astro2010 Decadal Survey. The authors welcome comments and discussion

A key obstacle to understanding the galaxy merger rate and its role in galaxy evolution is the difficulty in constraining the merger properties and time-scales from instantaneous snapshots of the real universe.The most common way to identify galaxy mergers is by morphology, yet current theoretical calculations of the time-scales for galaxy disturbances are quite crude. We present a morphological analysis of a large suite of GADGET N-Body/hydro-dynamical equal-mass gas-rich disc galaxy mergers which have been processed through the Monte-Carlo radiative transfer code SUNRISE. With the resulting images, we examine the dependence of quantitative morphology (G, M20, C, A) in the SDSS g-band on merger stage, dust, viewing angle, orbital parameters, gas properties, supernova feedback, and total mass. We find that mergers appear most disturbed in G-M20 and asymmetry at the first pass and at the final coalescence of their nuclei, but can have normal quantitative morphologies at other merger stages. The merger observability time-scales depend on the method used to identify the merger as well as the gas fraction, pericentric distance, and relative orientation of the merging galaxies. Enhanced star formation peaks after and lasts significantly longer than strong morphological disturbances. Despite their massive bulges, the majority of merger remnants appear disc-like and dusty in g-band light because of the presence of a low-mass star-forming disc., Comment: accepted to MNRAS; 29 pages, 19 color figures; go to http://www.noao.edu/staff/lotz/pdfs/lotz_arXiv.0805.1246.pdf for full resolution

Polar Ring Galaxies, such as NGC4650A, are a class of galaxy which have two kinematically distinct components that are inclined by almost 90 degrees to each other. These striking galaxies challenge our understanding of how galaxies form; the origin of their distinct components has remained uncertain, and the subject of much debate. We use high-resolution cosmological simulations of galaxy formation to show that Polar Ring Galaxies are simply an extreme example of the angular moment misalignment that occurs during the hierarchical structure formation characteristic of Cold Dark Matter cosmology. In our model, Polar Ring Galaxies form through the continuous accretion of gas whose angular momentum is misaligned with the central galaxy., Comment: Submitted to ApJ. 8 pages in emulate ApJ style. 2 associated animations are found at: http://www.youtube.com/watch?v=c-H3WzaewdY http://www.youtube.com/watch?v=9Xf3fJkgWEg

We employ numerical simulations of galaxy mergers to explore the effect of galaxy mass ratio on merger--driven starbursts. Our numerical simulations include radiative cooling of gas, star formation, and stellar feedback to follow the interaction and merger of four disk galaxies. The galaxy models span a factor of 23 in total mass and are designed to be representative of typical galaxies in the local Universe. We find that the merger--driven star formation is a strong function of merger mass ratio, with very little, if any, induced star formation for large mass ratio mergers. We define a burst efficiency that is useful to characterize the merger--driven star formation and test that it is insensitive to uncertainties in the feedback parameterization. In accord with previous work we find that the burst efficiency depends on the structure of the primary galaxy. In particular, the presence of a massive stellar bulge stabilizes the disk and suppresses merger--driven star formation for large mass ratio mergers. Direct, co--planar merging orbits produce the largest tidal disturbance and yield that most intense burst of star formation. Contrary to naive expectations, a more compact distribution of gas or an increased gas fraction both decrease the burst efficiency. Owing to the efficient feedback model and the newer version of SPH employed here, the burst efficiencies of the mergers presented here are smaller than in previous studies., Comment: 26 pages, 21 figures, submitted to MNRAS

We study the effects of dust in hydrodynamic simulations of spiral galaxies when different radial metallicity gradients are assumed. SUNRISE, a Monte-Carlo radiative-transfer code, is used to make detailed calculations of the internal extinction of disk galaxies caused by their dust content. SUNRISE is used on eight different Smooth Particle Hydrodynamics (SPH) simulations of isolated spiral galaxies. These galaxies vary mainly in mass and hence luminosity, spanning a range in luminosities from -16 to -22 magnitudes in the B band. We focus on the attenuation in different wavelength bands as a function of the disk inclination and the luminosity of the models, and compare this to observations. Observations suggest different metallicity gradients for galaxies of different luminosities. These metallicity gradients were explored in our different models, finding that the resulting dust attenuation matches observations for edge-on galaxies, but do not show a linear behaviour in log axis ratio as some observations have suggested. A quadratic law describing the dependence of attenuation on inclination, as proposed by more recent observations, reconciles the attenuation of the simulations at intermediate inclinations with observations. We also compare the total infrared-to-ultraviolet flux ratios for the simulated galaxies with those of the SINGS sample and find general agreement. Finally we compare our results with those from simpler models that do not take into account structure such as spiral arms, finding that the inclusion of sub-structure on the size scale of spiral arms does not change conclusions about the attenuation dependence on inclination or wavelength., Comment: 12 pages, 8 figures, all figures require colors, accepted for publication in MNRAS. Major changes from v1 include: Comparison of the calculated infrared-to-ultraviolet ratios in our models to those observed by SIGNS, clearer fonts in our plots and clearer justification of the rationale behind the adopted gradients. Changes from v2: Added journal-ref

Using a sample of 89 snapshots from 58 hydrodynamic binary galaxy major merger simulations, we find that stellar remnants are mostly oblate while dark matter halos are mostly prolate or triaxial. The stellar minor axis and the halo major axis are almost always nearly perpendicular. This can be understood by considering the influence of angular momentum and dissipation during the merger. If binary mergers of spiral galaxies are responsible for the formation of elliptical galaxies or some subpopulation thereof, these galaxies can be expected to be oblate and inhabit their halos with the predicted shapes and orientations. These predictions are potentially relevant to observational studies of weak gravitational lensing, where one must stack many optically aligned galaxies in order to determine the shape of the resulting stacked mass distribution. The simple relationship between the dark and luminous matter presented here can be used to guide the stacking of galaxies to minimize the information lost., Comment: 4 pages, 5 figures. Minor changes to match published version

This paper describes Sunrise, a parallel, free Monte-Carlo code for the calculation of radiation transfer through astronomical dust. Sunrise uses an adaptive-mesh refinement grid to describe arbitrary geometries of emitting and absorbing/scattering media, with spatial dynamical range exceeding 10^4, and it can efficiently generate images of the emerging radiation at arbitrary points in space. In addition to the monochromatic radiative transfer typically used by Monte-Carlo codes, Sunrise is capable of propagating a range of wavelengths simultaneously. This ``polychromatic'' algorithm gives significant improvements in efficiency and accuracy when spectral features are calculated. Sunrise is used to study the effects of dust in hydrodynamic simulations of interacting galaxies, and the procedure for this is described. The code is tested against previously published results., Comment: 22 pages, 11 figures, submitted to MNRAS. Changed based on referee response. Section on benchmarks expanded

Using hydrodynamic simulations of disc-galaxy major mergers, we investigate the star formation history and remnant properties when various parametrizations of a simple stellar feedback model are implemented. The simulations include radiative cooling, a density-dependent star formation recipe and a model for feedback from massive stars. The feedback model stores supernova feedback energy within individual gas particles and dissipates this energy on a time-scale specified by two free parameters; tau_fb, which sets the dissipative time-scale, and n, which sets the effective equation of state in star-forming regions. Using a self-consistent disc galaxy, modelled after a local Sbc spiral, in both isolated and major-merger simulations, we investigate parametrizations of the feedback model that are selected with respect to the quiescent disc stability. These models produce a range of star formation histories that are consistent with the star formation relation found by Kennicutt. All major mergers produce a population of new stars that is highly centrally concentrated, demonstrating a distinct break in the r1/4 surface density profile, consistent with previous findings. The half-mass radius and one-dimensional velocity dispersion are affected by the feedback model used. Finally, we compare our results to those of previous simulations of star formation in disc-galaxy major mergers, addressing the effects of star formation normalization, the version of smoothed particle hydrodynamics (SPH) employed and assumptions about the interstellar medium., Comment: update to match published version