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We present image-based evolution of galaxy mergers from the Illustris cosmological simulation at 12 time-steps over 0.5 < z < 5. To do so, we created approximately one million synthetic deep Hubble Space Telescope and James Webb Space Telescope images and measured common morphological indicators. Using the merger tree, we assess methods to observationally select mergers with stellar mass ratios as low as 10:1 completing within +/- 250 Myr of the mock observation. We confirm that common one- or two-dimensional statistics select mergers so defined with low purity and completeness, leading to high statistical errors. As an alternative, we train redshift-dependent random forests (RFs) based on 5-10 inputs. Cross-validation shows the RFs yield superior, yet still imperfect, measurements of the late-stage merger fraction, and they select more mergers in bulge-dominated galaxies. When applied to CANDELS morphology catalogs, the RFs estimate a merger rate increasing to at least z = 3, albeit two times higher than expected by theory. This suggests possible mismatches in the feedback-determined morphologies, but affirms the basic understanding of galaxy merger evolution. The RFs achieve completeness of roughly 70% at 0.5 < z < 3, and purity increasing from 10% at z = 0.5 to 60% at z = 3. At earlier times, the training sets are insufficient, motivating larger simulations and smaller time sampling. By blending large surveys and large simulations, such machine learning techniques offer a promising opportunity to teach us the strengths and weaknesses of inferences about galaxy evolution., 20 pages, 16 figures, MNRAS accepted version

At intermediate and high redshifts, measurements of galaxy star-formation rates are usually based on rest-frame ultraviolet (UV) data. A correction for dust attenuation, A_UV, is needed for these measurements. This correction is typically inferred from UV spectral slopes (beta) using an equation known as "Meurer's Relation." In this paper, we study this relation at a redshift of 1.5 using images and photometric measurements in the rest-frame UV (HST) through mid-infrared (Spitzer). It is shown that massive star-forming galaxies (above 10^10 Msun) have dust corrections that are dependent on their inclination to the line-of-sight. Edge-on galaxies have higher A_UV and infrared excess (IRX=L(IR)/L(UV)) than face-on galaxies at a given beta. Interestingly, dust corrections for low-mass star-forming galaxies do not depend on inclination. This is likely because more massive galaxies have more disk-like shapes/kinematics, while low-mass galaxies are more prolate and have more disturbed kinematics. To account for an inclination-dependent dust correction, a modified Meurer's Relation is derived: A_UV=4.43+1.99 beta - 1.73 (b/a-0.67), where b/a is the galaxy axis ratio. This inclination-dependence of A_UV can be explained by a two-component model of the dust distribution inside galaxies. In such a model, the dust attenuation of edge-on galaxies has a higher contribution from a "mixture" component (dust uniformly mixed with stars in the diffuse interstellar medium), and a lower contribution from a "birth cloud" component (near-spherical dust shells surrounding young stars in H II regions) than that of face-on galaxies. The difference is caused by the larger path-lengths through disks at higher inclinations., Comment: 25 pages, 14 figures. Accepted to ApJ. Figure 4 is the key plot

The rate of major galaxy-galaxy merging is theoretically predicted to steadily increase with redshift during the peak epoch of massive galaxy development ($1{\leq}z{\leq}3$). We use close-pair statistics to objectively study the incidence of massive galaxies (stellar $M_{1}{\geq}2{\times}10^{10}M_{\odot}$) hosting major companions ($1{\leq}M_{1}/M_{2}{\leq}4$; i.e., $$4:1) companions at $z>1$. We show that these evolutionary trends are statistically robust to changes in companion proximity. We find disagreements between published results are resolved when selection criteria are closely matched. If we compute merger rates using constant fraction-to-rate conversion factors ($C_{\mathrm{merg,pair}}{=}0.6$ and $T_{\mathrm{obs,pair}}{=}0.65\mathrm{Gyr}$), we find that MR rates disagree with theoretical predictions at $z{>}1.5$. Instead, if we use an evolving $T_{\mathrm{obs,pair}}(z){\propto}(1+z)^{-2}$ from Snyder et al., our MR-based rates agree with theory at $0{, Comment: 27 pages, 15 figures, 5 tables. Accepted for publication in Monthly Notices of the Royal Astronomical Society Main Journal

The role of galaxy mergers in fueling active galactic nuclei (AGN) is still debated, owing partly to selection effects inherent to studies of the merger/AGN connection. In particular, luminous AGN are often heavily obscured in late-stage mergers. Mid-infrared (IR) color selection of dust-enshrouded AGN with, e.g., the Wide-field Infrared Survey Explorer (WISE) has uncovered large new populations of obscured AGN. However, this method is sensitive mainly to AGN that dominate emission from the host. To understand how these selection biases affect mid-IR studies of the merger/AGN connection, we simulate the evolution of obscured AGN throughout galaxy mergers. Although mid-IR colors closely trace luminous, obscured AGN, we show that nearly half of merger-triggered AGN are missed with common mid-IR selection criteria, even in late-stage, gas-rich major mergers. At z < 0.5, where merger signatures and dual nuclei can most easily be detected, we find that a more lenient W1-W2 > 0.5 cut greatly improves completeness without significantly decreasing reliability. Extreme nuclear starbursts are briefly able to mimic this AGN signature, but this is largely irrelevant in mergers, where such starbursts are accompanied by AGN. We propose a two-color cut that yields high completeness and reliability even in starbursting systems. Further, we show that mid-IR color selection very effectively identifies dual AGN hosts, with the highest fraction at the smallest separations (< 3 kpc). Thus, many merger hosts of mid-IR AGN should contain unresolved dual AGN; these are ideal targets for high-resolution follow-up, particularly with the James Webb Space Telescope., 16 pages, 15 figures; published in MNRAS after minor revisions & some updated figures

We explore the evolution of the internal gas kinematics of star-forming galaxies from the peak of cosmic star-formation at $z\sim2$ to today. Measurements of galaxy rotation velocity $V_{rot}$, which quantify ordered motions, and gas velocity dispersion $\sigma_g$, which quantify disordered motions, are adopted from the DEEP2 and SIGMA surveys. This sample covers a continuous baseline in redshift from $z=2.5$ to $z=0.1$, spanning 10 Gyrs. At low redshift, nearly all sufficiently massive star-forming galaxies are rotationally supported ($V_{rot}>\sigma_g$). By $z=2$, the percentage of galaxies with rotational support has declined to 50$\%$ at low stellar mass ($10^{9}-10^{10}\,M_{\odot}$) and 70$\%$ at high stellar mass ($10^{10}-10^{11}M_{\odot}$). For $V_{rot}\,>\,3\,\sigma_g$, the percentage drops below 35$\%$ for all masses. From $z\,=\,2$ to now, galaxies exhibit remarkably smooth kinematic evolution on average. All galaxies tend towards rotational support with time, and it is reached earlier in higher mass systems. This is mostly due to an average decline in $\sigma_g$ by a factor of 3 since a redshift of 2, which is independent of mass. Over the same time period, $V_{rot}$ increases by a factor of 1.5 for low mass systems, but does not evolve for high mass systems. These trends in $V_{rot}$ and $\sigma_g$ with time are at a fixed stellar mass and should not be interpreted as evolutionary tracks for galaxy populations. When galaxy populations are linked in time with abundance matching, not only does $\sigma_g$ decline with time as before, but $V_{rot}$ strongly increases with time for all galaxy masses. This enhances the evolution in $V_{rot}/\sigma_g$. These results indicate that $z\,=\,2$ is a period of disk assembly, during which the strong rotational support present in today's massive disk galaxies is only just beginning to emerge., Comment: 12 pages, 8 figures, submitted to ApJ

We present two state-of-the-art models of the solar system, one corresponding to the present day and one to the Archean Eon 3.5 billion years ago. Each model contains spatial and spectral information for the star, the planets, and the interplanetary dust, extending to 50 AU from the sun and covering the wavelength range 0.3 to 2.5 micron. In addition, we created a spectral image cube representative of the astronomical backgrounds that will be seen behind deep observations of extrasolar planetary systems, including galaxies and Milky Way stars. These models are intended as inputs to high-fidelity simulations of direct observations of exoplanetary systems using telescopes equipped with high-contrast capability. They will help improve the realism of observation and instrument parameters that are required inputs to statistical observatory yield calculations, as well as guide development of post-processing algorithms for telescopes capable of directly imaging Earth-like planets., Comment: Accepted for publication in PASP

We present a detailed, multi-wavelength study of star formation (SF) and AGN activity in 11 near-infrared (IR) selected, spectroscopically confirmed, massive ($\gtrsim10^{14}\,\rm{M_{\odot}}$) galaxy clusters at $1, Comment: 26 pages, 14 figures, 6 tables with appendix, accepted for publication in the Astrophysical Journal

We compare mass-selected close pairs at z > 1 with the intrinsic galaxy merger rate in the Illustris Simulations. To do so, we construct three 140 arcmin^2 lightcone catalogs and measure pair fractions, finding that they change little or decrease with increasing redshift at z > 1. Consistent with current surveys, this trend requires a decrease in the merger-pair observability time, roughly as (1 + z)^-2, in order to measure the merger rates of the same galaxies. This implies that major mergers are more common at high redshift than implied by the simplest arguments assuming a constant observability time. Several effects contribute to this trend: (1) The fraction of massive, major (4:1) pairs which merge by today increases weakly from ~0.5 at z=1 to ~0.8 at z=3. (2) The median time elapsed between an observed pair and final remnant decreases by a factor of two from z~1 to z~3. (3) An increasing specific star formation rate (sSFR) decreases the time during which common stellar-mass based pair selection criteria could identify the mergers. The average orbit of the pairs at observation time varies only weakly, suggesting that the dynamical time is not varying enough to account by itself for the pair fraction trends. Merging pairs reside in dense regions, having overdensity ~10 to ~100 times greater than the average massive galaxy. We forward model the pairs to reconstruct the merger remnant production rate, showing that it is consistent with a rapid increase in galaxy merger rates at z > 1., MNRAS accepted version, 11 pages, 10 figures

Mergers and the spin of the dark matter halo are factors traditionally believed to determine the morphology of galaxies within a $\Lambda$CDM cosmology. We study this hypothesis by considering approximately 18,000 central galaxies at $z=0$ with stellar masses $M_{\ast} = 10^{9}-10^{12} \, {\rm M}_{\odot}$ selected from the Illustris cosmological hydrodynamic simulation. The fraction of accreted stars -- which measures the importance of massive, recent and dry mergers -- increases steeply with galaxy stellar mass, from less than 5 per cent in dwarfs to 80 per cent in the most massive objects, and the impact of mergers on galaxy morphology increases accordingly. For galaxies with $M_{\ast} \gtrsim 10^{11} \, {\rm M}_{\odot}$, mergers have the expected effect: if gas-poor they promote the formation of spheroidal galaxies, whereas gas-rich mergers favour the formation and survivability of massive discs. This trend, however, breaks at lower masses. For objects with $M_{\ast} \lesssim 10^{11} \, {\rm M}_{\odot}$, mergers do not seem to play any significant role in determining the morphology, with accreted stellar fractions and mean merger gas fractions that are indistinguishable between spheroidal and disc-dominated galaxies. On the other hand, halo spin correlates with morphology primarily in the least massive objects in the sample ($M_{\ast} \lesssim 10^{10} \, {\rm M}_{\odot}$), but only weakly for galaxies above that mass. Our results support a scenario where (1) mergers play a dominant role in shaping the morphology of massive galaxies, (2) halo spin is important for the morphology of dwarfs, and (3) the morphology of medium-sized galaxies -- including the Milky Way -- shows little dependence on galaxy assembly history or halo spin, at least when these two factors are considered individually., Comment: 16 pages, 14 figures, accepted for publication in MNRAS

We present the full public release of all data from the Illustris simulation project. Illustris is a suite of large volume, cosmological hydrodynamical simulations run with the moving-mesh code Arepo and including a comprehensive set of physical models critical for following the formation and evolution of galaxies across cosmic time. Each simulates a volume of (106.5 Mpc)^3 and self-consistently evolves five different types of resolution elements from a starting redshift of z=127 to the present day, z=0. These components are: dark matter particles, gas cells, passive gas tracers, stars and stellar wind particles, and supermassive black holes. This data release includes the snapshots at all 136 available redshifts, halo and subhalo catalogs at each snapshot, and two distinct merger trees. Six primary realizations of the Illustris volume are released, including the flagship Illustris-1 run. These include three resolution levels with the fiducial "full" baryonic physics model, and a dark matter only analog for each. In addition, we provide four distinct, high time resolution, smaller volume "subboxes". The total data volume is ~265 TB, including ~800 full volume snapshots and ~30,000 subbox snapshots. We describe the released data products as well as tools we have developed for their analysis. All data may be directly downloaded in its native HDF5 format. Additionally, we release a comprehensive, web-based API which allows programmatic access to search and data processing tasks. In both cases we provide example scripts and a getting-started guide in several languages: currently, IDL, Python, and Matlab. This paper addresses scientific issues relevant for the interpretation of the simulations, serves as a pointer to published and on-line documentation of the project, describes planned future additional data releases, and discusses technical aspects of the release., The data is made available at http://www.illustris-project.org/data/ (comments welcome)

I discuss recent work in which we construct models of poststarburst galaxies by combining fully three-dimensional hydrodynamic simulations of galaxy mergers with radiative transfer calculations of dust attenuation. The poststarburst signatures can occur shortly after a bright starburst phase in gas-rich mergers, and thus offer a unique opportunity to study the formation of bulges and the effects of feedback. Several additional applications of spatially-resolved spectroscopic models of interacting galaxies include multi-wavelength studies of AGN/starburst diagnostics, mock integral field unit data to interpret the evolution of ULIRGs, and the ‘Green Valley’.Optical spectra of simulated major gas-rich galaxy mergers can be found at http://www.cfa.harvard.edu/~gsnyder

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved, From mock Hubble Space Telescope images, we quantify non-parametric statistics of galaxy morphology, thereby predicting the emergence of relationships among stellar mass, star formation, and observed rest-frame optical structure at 1 < z < 3. We measure automated diagnostics of galaxy morphology in cosmological simulations of the formation of 22 central galaxies with 9.3 < log10 M_*/M_sun < 10.7. These high-spatial-resolution zoom-in calculations enable accurate modeling of the rest-frame UV and optical morphology. Even with small numbers of galaxies, we find that structural evolution is neither universal nor monotonic: galaxy interactions can trigger either bulge or disc formation, and optically bulge-dominated galaxies at this mass may not remain so forever. Simulated galaxies with M_* > 10^10 M_sun contain relatively more disc-dominated light profiles than those with lower mass, reflecting significant disc brightening in some haloes at 1 < z < 2. By this epoch, simulated galaxies with specific star formation rates below 10^-9.7 yr^-1 are more likely than normal star-formers to have a broader mix of structural types, especially at M_* > 10^10 M_sun. We analyze a cosmological major merger at z ~ 1.5 and find that the newly proposed MID morphology diagnostics trace later merger stages while G-M20 trace earlier ones. MID is sensitive also to clumpy star-forming discs. The observability time of typical MID-enhanced events in our simulation sample is less than 100 Myr. A larger sample of cosmological assembly histories may be required to calibrate such diagnostics in the face of their sensitivity to viewing angle, segmentation algorithm, and various phenomena such as clumpy star formation and minor mergers, This work was partially supported by MINECO-AYA2012-31101. DC is a Juan de la Cierva fellow. This research has been partly supported by ISF grant 24/12, by NSF grant AST-1010033, and by the I-CORE Program of the PBC and the ISF grant 1829/12. GS and JL appreciate support from the HST grants program, number HST-AR-12856:01-A. Support for program #12856 (PI J. Lotz) was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555

Massive, quiescent galaxies at high redshift have been found to be considerably more compact than galaxies of similar mass in the local universe. How these compact galaxies formed has yet to be determined, though several progenitor populations have been proposed. Here we investigate the formation processes and quantify the assembly histories of such galaxies in Illustris, a suite of hydrodynamical cosmological simulations encompassing a sufficiently large volume to include rare objects, while simultaneously resolving the internal structure of galaxies. We select massive (~10^11 solar masses) and compact (stellar half-mass radius < 2 kpc) galaxies from the simulation at z=2. Within the Illustris suite, we find that these quantities are not perfectly converged, but are reasonably reliable for our purposes. The resulting population is composed primarily of quiescent galaxies, but we also find several star-forming compact galaxies. The simulated compact galaxies are similar to observed galaxies in star formation activity and appearance. We follow their evolution at high redshift in the simulation and find that there are multiple pathways to form these compact galaxies, dominated by two mechanisms: (i) intense, centrally concentrated starbursts generally triggered by gas-rich major mergers between z~2-4, reducing the galaxies' half-mass radii by a factor of a few to below 2 kpc, and (ii) assembly at very early times when the universe was much denser; the galaxies formed compact and remained so until z~2., 13 pages, 10 figures, MNRAS accepted version

We present our methods for generating a catalog of 7,000 synthetic images and 40,000 integrated spectra of redshift z = 0 galaxies from the Illustris Simulation. The mock data products are produced by using stellar population synthesis models to assign spectral energy distributions (SED) to each star particle in the galaxies. The resulting synthetic images and integrated SEDs therefore properly reflect the spatial distribution, stellar metallicity distribution, and star formation history of the galaxies. From the synthetic data products it is possible to produce monochromatic or color-composite images, perform SED fitting, classify morphology, determine galaxy structural properties, and evaluate the impacts of galaxy viewing angle. The main contribution of this paper is to describe the production, format, and composition of the image catalog that makes up the Illustris Simulation Obsevatory. As a demonstration of this resource, we derive galactic stellar mass estimates by applying the SED fitting code FAST to the synthetic galaxy products, and compare the derived stellar masses against the true stellar masses from the simulation. We find from this idealized experiment that systematic biases exist in the photometrically derived stellar mass values that can be reduced by using a fixed metallicity in conjunction with a minimum galaxy age restriction., Comment: 21 pages, 17 figures, submitted to MNRAS. Comments welcome

Supermassive black hole (BH) mergers produce powerful gravitational wave (GW) emission. Asymmetry in this emission imparts a recoil kick to the merged BH, which can eject the BH from its host galaxy altogether. Recoiling BHs could be observed as offset active galactic nuclei (AGN). Several candidates have been identified, but systematic searches have been hampered by large uncertainties regarding their observability. By extracting merging BHs and host galaxy properties from the Illustris cosmological simulations, we have developed a comprehensive model for recoiling AGN. Here, for the first time, we model the effects of BH spin alignment and recoil dynamics based on the gas-richness of host galaxies. We predict that if BH spins are not highly aligned, seeing-limited observations could resolve offset AGN, making them promising targets for all-sky surveys. For randomly-oriented spins, less than about 10 spatially-offset AGN may be detectable in HST-COSMOS, and > 10^3 could be found with Pan-STARRS, LSST, Euclid, and WFIRST. Nearly a thousand velocity-offset AGN are predicted within the SDSS footprint; the rarity of large broad-line offsets among SDSS quasars is likely due in part to selection effects but suggests that spin alignment plays a role in suppressing recoils. Nonetheless, in our most physically motivated model where alignment occurs only in gas-rich mergers, hundreds of offset AGN should be found in all-sky surveys. Our findings strongly motivate a dedicated search for recoiling AGN., 30 pages, 19 figures. Accepted to MNRAS after minor revisions

We introduce the Illustris Project, a series of large-scale hydrodynamical simulations of galaxy formation. The highest resolution simulation, Illustris-1, covers a volume of $(106.5\,{\rm Mpc})^3$, has a dark mass resolution of ${6.26 \times 10^{6}\,{\rm M}_\odot}$, and an initial baryonic matter mass resolution of ${1.26 \times 10^{6}\,{\rm M}_\odot}$. At $z=0$ gravitational forces are softened on scales of $710\,{\rm pc}$, and the smallest hydrodynamical gas cells have an extent of $48\,{\rm pc}$. We follow the dynamical evolution of $2\times 1820^3$ resolution elements and in addition passively evolve $1820^3$ Monte Carlo tracer particles reaching a total particle count of more than $18$ billion. The galaxy formation model includes: primordial and metal-line cooling with self-shielding corrections, stellar evolution, stellar feedback, gas recycling, chemical enrichment, supermassive black hole growth, and feedback from active galactic nuclei. At $z=0$ our simulation volume contains about $40,000$ well-resolved galaxies covering a diverse range of morphologies and colours including early-type, late-type and irregular galaxies. The simulation reproduces reasonably well the cosmic star formation rate density, the galaxy luminosity function, and baryon conversion efficiency at $z=0$. It also qualitatively captures the impact of galaxy environment on the red fractions of galaxies. The internal velocity structure of selected well-resolved disk galaxies obeys the stellar and baryonic Tully-Fisher relation together with flat circular velocity curves. In the well-resolved regime the simulation reproduces the observed mix of early-type and late-type galaxies. Our model predicts a halo mass dependent impact of baryonic effects on the halo mass function and the masses of haloes caused by feedback from supernova and active galactic nuclei., 32 pages, 26 figures. MNRAS accepted. The official Illustris website can be found at http://www.illustris-project.org

We measure the star formation rates (SFRs) of massive ($M_{\star}>10^{10.1}M_{\odot}$) early-type galaxies (ETGs) in a sample of 11 high-redshift ($1.0 < z < 1.5$) galaxy clusters drawn from the IRAC Shallow Cluster Survey (ISCS). We identify ETGs visually from Hubble Space Telescope imaging and select likely cluster members as having either an appropriate spectroscopic redshift or red sequence color. Mid-infrared SFRs are measured using Spitzer 24 $��$m data for isolated cluster galaxies for which contamination by neighbors, and active galactic nuclei, can be ruled out. Cluster ETGs show enhanced specific star formation rates (sSFRs) compared to cluster galaxies in the local Universe, but have sSFRs more than four times lower than that of field ETGs at $1 < z < 1.5$. Relative to the late-type cluster population, isolated ETGs show substantially quenched mean SFRs, yet still contribute 12% of the overall star formation activity measured in $1 < z < 1.5$ clusters. We find that new ETGs are likely being formed in ISCS clusters; the fraction of cluster galaxies identified as ETGs increases from 34% to 56% from $z \sim 1.5 \rightarrow 1.25$. While the fraction of cluster ETGs that are highly star-forming ($\textrm{SFR}\geq26\ M_{\odot}$ yr$^{-1}$) drops from 27% to 10% over the same period, their sSFRs are roughly constant. All these factors taken together suggest that, particularly at $z\gtrsim1.25$, the events that created these distant cluster ETGs$-$likely mergers, at least among the most massive$-$were both recent and gas-rich., 12 pages, 8 figures. Accepted for publication in The Astrophysical Journal

We study the properties of black holes and their host galaxies across cosmic time in the Illustris simulation. Illustris is a large scale cosmological hydrodynamical simulation which resolves a (106.5 Mpc)^3 volume with more than 12 billion resolution elements and includes state-of-the-art physical models relevant for galaxy formation. We find that the black hole mass density for redshifts z = 0 - 5 and the black hole mass function at z = 0 predicted by Illustris are in very good agreement with the most recent observational constraints. We show that the bolometric and hard X-ray luminosity functions of AGN at z = 0 and 1 reproduce observational data very well over the full dynamic range probed. Unless the bolometric corrections are largely underestimated, this requires radiative efficiencies to be on average low, epsilon_r, 24 pages, 15 figures. MNRAS accepted. The Illustris website can be found at http://www.illustris-project.org/

We develop and test an automated technique to model the dynamics of interacting galaxy pairs. We use Identikit (Barnes & Hibbard 2009, Barnes 2011) as a tool for modeling and matching the morphology and kinematics of the interacting pairs of equal-mass galaxies. In order to reduce the effect of subjective human judgement, we automate the selection of phase-space regions used to match simulations to data, and we explore how selection of these regions affects the random uncertainties of parameters in the best-fit model. In this work, we use an independent set of GADGET SPH simulations as input data to determine the systematic bias in the measured encounter parameters based on the known initial conditions of these simulations. We test both cold gas and young stellar components in the GADGET simulations to explore the effect of choosing HI vs. H$\alpha$ as the line of sight velocity tracer. We find that we can group the results into tests with good, fair, and poor convergence based on the distribution of parameters of models close to the best-fit model. For tests with good and fair convergence, we rule out large fractions of parameter space and recover merger stage, eccentricity, pericentric distance, viewing angle, and initial disc orientations within 3$\sigma$ of the correct value. All of tests on prograde-prograde systems have either good or fair convergence. The results of tests on edge-on discs are less biased than face-on tests. Retrograde and polar systems do not converge and may require constraints from regions other than the tidal tails and bridges., Comment: 18 pages, 13 figures, 3 Tables, Accepted for publication in MNRAS

Previous simulations of the growth of cosmic structures have broadly reproduced the 'cosmic web' of galaxies that we see in the Universe, but failed to create a mixed population of elliptical and spiral galaxies due to numerical inaccuracies and incomplete physical models. Moreover, because of computational constraints, they were unable to track the small scale evolution of gas and stars to the present epoch within a representative portion of the Universe. Here we report a simulation that starts 12 million years after the Big Bang, and traces 13 billion years of cosmic evolution with 12 billion resolution elements in a volume of $(106.5\,{\rm Mpc})^3$. It yields a reasonable population of ellipticals and spirals, reproduces the distribution of galaxies in clusters and statistics of hydrogen on large scales, and at the same time the metal and hydrogen content of galaxies on small scales., Comment: 32 pages, 5 figures, Nature Article (May 8, 2014). The official Illustris website can be found at http://www.illustris-project.org

We analyze the star formation properties of 16 infrared-selected, spectroscopically confirmed galaxy clusters at $1 < z < 1.5$ from the Spitzer/IRAC Shallow Cluster Survey (ISCS). We present new spectroscopic confirmation for six of these high-redshift clusters, five of which are at $z>1.35$. Using infrared luminosities measured with deep Spitzer/MIPS observations at 24 $��$m, along with robust optical+IRAC photometric redshifts and SED-fitted stellar masses, we present the dust-obscured star-forming fractions, star formation rates and specific star formation rates in these clusters as functions of redshift and projected clustercentric radius. We find that $z\sim 1.4$ represents a transition redshift for the ISCS sample, with clear evidence of an unquenched era of cluster star formation at earlier times. Beyond this redshift the fraction of star-forming cluster members increases monotonically toward the cluster centers. Indeed, the specific star formation rate in the cores of these distant clusters is consistent with field values at similar redshifts, indicating that at $z>1.4$ environment-dependent quenching had not yet been established in ISCS clusters. Combining these observations with complementary studies showing a rapid increase in the AGN fraction, a stochastic star formation history, and a major merging episode at the same epoch in this cluster sample, we suggest that the starburst activity is likely merger-driven and that the subsequent quenching is due to feedback from merger-fueled AGN. The totality of the evidence suggests we are witnessing the final quenching period that brings an end to the era of star formation in galaxy clusters and initiates the era of passive evolution., 16 pages, 8 figures, 3 tables. Accepted for publication in The Astrophysical Journal

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., 21 pages, 13 figures, ApJ accepted version

We compare the star formation (SF) activity in cluster galaxies to the field from z=0.3-1.5 using $Herschel$ SPIRE 250$��$m imaging. We utilize 274 clusters from the IRAC Shallow Cluster Survey (ISCS) selected as rest-frame near-infrared overdensities over the 9 square degree Bootes field . This analysis allows us to quantify the evolution of SF in clusters over a long redshift baseline without bias against active cluster systems. Using a stacking analysis, we determine the average star formation rates (SFRs) and specific-SFRs (SSFR=SFR/M$_{\star}$) of stellar mass-limited (M>1.3x10$^{10}$ M$_{\odot}$), statistical samples of cluster and field galaxies, probing both the star forming and quiescent populations. We find a clear indication that the average SF in cluster galaxies is evolving more rapidly than in the field, with field SF levels at z>1.2 in the cluster cores (r0.5 Mpc). These general trends in the cluster cores and outskirts are driven by the lower mass galaxies in our sample. Blue cluster galaxies have systematically lower SSFRs than blue field galaxies, but otherwise show no strong differential evolution with respect to the field over our redshift range. This suggests that the cluster environment is both suppressing the star formation in blue galaxies on long time-scales and rapidly transitioning some fraction of blue galaxies to the quiescent galaxy population on short time-scales. We argue that our results are consistent with both strangulation and ram pressure stripping acting in these clusters, with merger activity occurring in the cluster outskirts., 23 pages, 11 figures, 5 tables. Accepted for publication in MNRAS

We present results for the assembly and star formation histories of massive (~L*) red sequence galaxies in 11 spectroscopically confirmed, infrared-selected galaxy clusters at 1.0 < z < 1.5, the precursors to present-day massive clusters with M ~ 10^15 M_sun. Using rest-frame optical photometry, we investigate evolution in the color and scatter of the red sequence galaxy population, comparing with models of possible star formation histories. In contrast to studies of central cluster galaxies at lower redshift (z < 1), these data are clearly inconsistent with the continued evolution of stars formed and assembled primarily at a single, much-earlier time. Specifically, we find that the colors of massive cluster galaxies at z = 1.5 imply that the bulk of star formation occurred at z ~ 3, whereas by z = 1 their colors imply formation at z ~ 2; therefore these galaxies exhibit approximately the same luminosity-weighted stellar age at 1 < z < 1.5. This likely reflects star formation that occurs over an extended period, the effects of significant progenitor bias, or both. Our results generally indicate that massive cluster galaxy populations began forming a significant mass of stars at z >~ 4, contained some red spheroids by z ~ 1.5, and were actively assembling much of their final mass during 1 < z < 2 in the form of younger stars. Qualitatively, the slopes of the cluster color-magnitude relations are consistent with no significant evolution relative to local clusters., 24 pages, 9 figures, accepted to ApJ

We report 31 GHz CARMA observations of IDCS J1426.5+3508, an infrared-selected galaxy cluster at z = 1.75. A Sunyaev-Zel'dovich decrement is detected towards this cluster, indicating a total mass of M200 = (4.3 +/- 1.1) x 10^{14} Msun in agreement with the approximate X-ray mass of ~5 x 10^{14} Msun. IDCS J1426.5+3508 is by far the most distant cluster yet detected via the Sunyaev-Zel'dovich effect, and the most massive z >= 1.4 galaxy cluster found to date. Despite the mere ~1% probability of finding it in the 8.82 deg^2 IRAC Distant Cluster Survey, IDCS J1426.5+3508 is not completely unexpected in LCDM once the area of large, existing surveys is considered. IDCS J1426.5+3508 is, however, among the rarest, most extreme clusters ever discovered, and indeed is an evolutionary precursor to the most massive known clusters at all redshifts. We discuss how imminent, highly sensitive Sunyaev-Zel'dovich experiments will complement infrared techniques for statistical studies of the formation of the most massive galaxy clusters in the z > 1.5 Universe, including potential precursors to IDCS J1426.5+3508., 9 pages, 4 figures, Accepted to the Astrophysical Journal

The magnetohydrodynamic evolution of a dense spherical cloud as it interacts with a strong planar shock is studied, as a model for shock interactions with density inhomogeneities in the interstellar medium. The cloud is assumed to be small enough that radiative cooling, thermal conduction, and self-gravity can be ignored. A variety of initial orientations (including parallel, perpendicular, and oblique to the incident shock normal) and strengths for the magnetic field are investigated. During the early stages of the interaction (less than twice the time taken for the transmitted shock to cross the interior of the cloud) the structure and dynamics of the shocked cloud is fairly insensitive to the magnetic field strength and orientation. However, at late times strong fields substantially alter the dynamics of the cloud, suppressing fragmentation and mixing by stabilizing the interface at the cloud surface. Even weak magnetic fields can drastically alter the evolution of the cloud compared to the hydrodynamic case. Weak fields of different geometries result in different distributions and amplifications of the magnetic energy density, which may affect the thermal and non-thermal x-ray emission expected from shocked clouds associated with, for example, supernovae remnants., Accepted for publication in Astrophysical Journal; a higher resolution file can be found at http://www.astro.princeton.edu/~msshin/science/shock_cloud.pdf.gz

We report the discovery of an IR-selected galaxy cluster in the IRAC Distant Cluster Survey (IDCS). New data from the Hubble Space Telescope spectroscopically confirm IDCS J1433.2+3306 at z = 1.89 with robust spectroscopic redshifts for seven members, two of which are based on the 4000 A break. Detected emission lines such as [O II] and H{beta} indicate star formation rates of {approx}>20 M{sub Sun} yr{sup -1} for three galaxies within a 500 kpc projected radius of the cluster center. The cluster exhibits a red sequence with a scatter and color indicative of a formation redshift z{sub f} {approx}> 3.5. The stellar age of the early-type galaxy population is approximately consistent with those of clusters at lower redshift (1

We report the discovery of an IR-selected massive galaxy cluster in the IRAC Distant Cluster Survey (IDCS). We present new data from the Hubble Space Telescope and the W. M. Keck Observatory that spectroscopically confirm IDCS J1426+3508 at z=1.75. Moreover, the cluster is detected in archival Chandra data as an extended X-ray source, comprising 54 counts after the removal of point sources. We calculate an X-ray luminosity of L{0.5-2 keV} = (5.5 +/- 1.2) X 1e44 ergs/s within r = 60 arcsec (~1 Mpc diameter), which implies M_{200,L_x} = (5.6 +/- 1.6) X 1e14 Msun. IDCS J1426+3508 appears to be an exceptionally massive cluster for its redshift.

We analyze the relation between the mass of the central supermassive black hole (Mbh) and the number of globular clusters (Ngc) in elliptical galaxies and bulges as a ramification of the black hole fundamental plane, the theoretically predicted and observed multi-variable correlation between Mbh and bulge binding energy. Although the tightness of the Mbh-Ngc correlation suggests an unlikely causal link between supermassive black holes and globular clusters, such a correspondence can exhibit small scatter even if the physical relationship is indirect. We show that the relatively small scatter of the Mbh-Ngc relation owes to the mutual residual correlation of Mbh and Ngc with stellar mass when the velocity dispersion is held fixed. Thus, present observations lend evidence for feedback-regulated models in which the bulge binding energy is most important; they do not necessarily imply any `special' connection between globular clusters and Mbh. This raises the question of why Ngc traces the formation of ellipticals and bulges sufficiently well to be correlated with binding energy., Comment: 7 pages, 2 figures, accepted for publication in ApJL