arXiv subjects

Philip F. Hopkins

Publications and source records attributed to Philip F. Hopkins.

At least 397 records · Page 22Linked to original sources

arXiv2008

The Radical Consequences of Realistic Satellite Orbits for the Heating and Implied Merger Histories of Galactic Disks

Previous models of galactic disk heating in interactions invoke restrictive assumptions not necessarily valid in modern LCDM contexts: that satellites and orbits are rigid and circular, with slow decay over many orbital times from dynamical friction. This leads to a linear scaling of disk heating with satellite mass: disk heights and velocity dispersions scale ~M_sat/M_disk. In turn, observed disk thicknesses present strong constraints on merger histories: the implication for the Milky Way is that <5% of its mass could come from mergers since z~2, in conflict with cosmological predictions. More realistically, satellites merge on nearly radial orbits, and once near the disk, resonant interactions efficiently remove angular momentum while tidal effects strip mass, leading to rapid merger/destruction in a couple of free-fall plunges. Under these conditions the proper heating efficiency is non-linear in mass ratio, ~(M_sat/M_disk)^2. We derive the scaling of disk scale heights and velocity dispersions as a function of mass ratio and disk gas content in this regime, and show this accurately describes the results of simulations with proper 'live' halos and disks. Under realistic circumstances, disk heating in minor mergers is suppressed by an order of magnitude relative to expectations of previous models. We show that the Milky Way disk could have absorbed ~5-10 1:10 mass-ratio mergers since z=2, in agreement with cosmological models. These distinctions lead to dramatic differences in which mass ratios are most important for disk heating and in the isophotal shapes of disk+bulge systems.

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arXiv2008

A Semi-Analytic Model for the Co-evolution of Galaxies, Black Holes, and Active Galactic Nuclei

We present a new semi-analytic model that self-consistently traces the growth of supermassive black holes (BH) and their host galaxies within the context of the LCDM cosmological framework. In our model, the energy emitted by accreting black holes regulates the growth of the black holes themselves, drives galactic scale winds that can remove cold gas from galaxies, and produces powerful jets that heat the hot gas atmospheres surrounding groups and clusters. We present a comprehensive comparison of our model predictions with observational measurements of key physical properties of low-redshift galaxies, such as cold gas fractions, stellar metallicities and ages, and specific star formation rates. We find that our new models successfully reproduce the exponential cutoff in the stellar mass function and the stellar and cold gas mass densities at z~0, and predict that star formation should be largely, but not entirely, quenched in massive galaxies at the present day. We also find that our model of self-regulated BH growth naturally reproduces the observed relation between BH mass and bulge mass. We explore the global formation history of galaxies in our models, presenting predictions for the cosmic histories of star formation, stellar mass assembly, cold gas, and metals. We find that models assuming the "concordance" LCDM cosmology overproduce star formation and stellar mass at high redshift (z>2). A model with less small-scale power predicts less star formation at high redshift, and excellent agreement with the observed stellar mass assembly history, but may have difficulty accounting for the cold gas in quasar absorption systems at high redshift (z~3-4).

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arXiv2008

Dissipation and the Fundamental Plane: Observational Tests

We develop observational tests of the idea that dissipation in gas-rich mergers produces the fundamental plane (FP) and related correlations obeyed by ellipticals. The FP 'tilt' implies lower-mass ellipticals have a higher ratio of stellar to dark matter within their stellar effective radii. Models argue that mergers between more gas-rich (typically lower-mass) disks yield larger mass fractions formed in compact starbursts, giving a smaller stellar R_e and higher M_stellar/M_tot within that R_e. Such starbursts leave a characteristic imprint in the surface brightness profile: a central excess above an outer profile established by the dissipationless violent relaxation of disk stars. In previous work, we developed empirical methods to decompose the observed profiles of ellipticals and robustly estimate the amount of dissipation in the original spheroid-forming merger(s). Applying this to a large sample of observed ellipticals, we test whether or not their location on the FP and its tilt are driven by dissipation. At fixed mass, ellipticals formed in more dissipational events are smaller and have higher M_stellar/M_tot. At fixed degree of dissipation, there is no tilt in the FP. We show that the dynamical mass estimator R_e*sigma^2/G is a good estimator of the true mass: the observed FP tilt cannot primarily owe to other forms of non-homology. Removing the effects of dissipation, observed ellipticals obey the same FP correlations as disks: unusual progenitors are not required to make typical ellipticals. Dissipation appears to be both necessary and sufficient to explain the FP tilt.

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arXiv2008

The Self-Regulated Growth of Supermassive Black Holes

We present a series of simulations of the self--regulated growth of supermassive black holes (SMBHs) in galaxies via three different fueling mechanisms: major mergers, minor mergers, and disk instabilities. The SMBHs in all three scenarios follow the same black hole fundamental plane (BHFP) and correlation with bulge binding energy seen in simulations of major mergers, and observed locally. Furthermore, provided that the total gas supply is significantly larger than the mass of the SMBH, its limiting mass is not influenced by the amount of gas available or the efficiency of black hole growth. This supports the assertion that SMBHs accrete until they reach a critical mass at which feedback is sufficient to unbind the gas locally, terminating the inflow and stalling further growth. At the same time, while minor and major mergers follow the same projected correlations (e.g., the $M_{BH}-σ$ and Magorrian relations), SMBHs grown via disk instabilities do not, owing to structural differences between the host bulges. This finding is supported by recent observations of SMBHs in pseudobulges and bulges in barred systems, as compared to those hosted by classical bulges. Taken together, this provides support for the BHFP and binding energy correlations as being more "fundamental" than other proposed correlations in that they reflect the physical mechanism driving the co-evolution of SMBHs and spheroids.

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arXiv2008

Dissipation and Extra Light in Galactic Nuclei: I. Gas-Rich Merger Remnants

We study the origin and properties of 'extra' or 'excess' central light in the surface brightness profiles of gas-rich merger remnants. Combining a large set of hydrodynamical simulations with data on observed mergers (spanning a broad range of profiles at various masses and degrees of relaxation), we show how to robustly separate the physically meaningful extra light -- stellar populations formed in a compact central starburst during a gas-rich merger -- from the outer profile established by violent relaxation acting on stars already present in the progenitors prior to the final merger. This separation is sensitive to the profile treatment, and we demonstrate that certain fitting procedures can yield physically misleading results. We show that our method reliably recovers the younger starburst population, and examine how the properties of this component scale with mass, gas content, and other aspects of the progenitors. We consider the time evolution of profiles in different bands, and estimate biases introduced by observational studies at different times and wavelengths. We show that extra light is ubiquitous in observed and simulated gas-rich merger remnants, with sufficient mass (~3-30% of the stellar mass) to explain the discrepancy in the maximum phase-space densities of ellipticals and their progenitor spirals. The nature of this central component provides powerful new constraints on the formation histories of observed systems.

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arXiv2008

Modeling the Dust Properties of z ~ 6 Quasars with ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree

The detection of large quantities of dust in z ~ 6 quasars by infrared and radio surveys presents puzzles for the formation and evolution of dust in these early systems. Previously (Li et al. 2007), we showed that luminous quasars at z > 6 can form through hierarchical mergers of gas-rich galaxies. Here, we calculate the dust properties of simulated quasars and their progenitors using a three-dimensional Monte Carlo radiative transfer code, ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree. ART^2 incorporates a radiative equilibrium algorithm for dust emission, an adaptive grid for inhomogeneous density, a multiphase model for the ISM, and a supernova-origin dust model. We reproduce the SED and dust properties of SDSS J1148+5251, and find that the infrared emission are closely associated with the formation and evolution of the quasar host. The system evolves from a cold to a warm ULIRG owing to heating and feedback from stars and AGN. Furthermore, the AGN has significant implications for the interpretation of observation of the hosts. Our results suggest that vigorous star formation in merging progenitors is necessary to reproduce the observed dust properties of z~6 quasars, supporting a merger-driven origin for luminous quasars at high redshifts and the starburst-to-quasar evolutionary hypothesis. (Abridged)

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arXiv2007

Rest-Frame Ultraviolet to Near Infrared Observations of an Interacting Lyman Break Galaxy at z = 4.42

We present the rest-frame ultraviolet through near infrared spectral energy distribution for an interacting Lyman break galaxy at a redshift z=4.42, the highest redshift merging system known with clearly resolved tidal features. The two objects in this system - HDF-G4 and its previously unidentified companion - are both B_{435} band dropouts, have similar V_{606}-i_{775} and i_{775}-z_{850} colors, and are separated by 1", which at z=4.42 corresponds to 7 kpc projected nuclear separation; all indicative of an interacting system. Fits to stellar population models indicate a stellar mass of M_\star = 2.6\times 10^{10} M_\odot, age of τ_\star = 720 My, and exponential star formation history with an e-folding time τ_0 = 440 My. Using these derived stellar populations as constraints, we model the HDF-G4 system using hydrodynamical simulations, and find that it will likely evolve into a quasar by z\sim3.5, and a quiescent, compact spheroid by z\sim 2.5 similar to those observed at z > 2. And, the existence of such an object supports galaxy formation models in which major mergers drive the high redshift buildup of spheroids and black holes.

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arXiv2007

A Theoretical Interpretation of the Black Hole Fundamental Plane

We examine the origin and evolution of correlations between properties of supermassive black holes (BHs) and their host galaxies using simulations of major galaxy mergers, including the effects of gas dissipation, cooling, star formation, and BH accretion and feedback. We demonstrate that the simulations predict the existence of a BH 'fundamental plane' (BHFP), of the form M_BH sigma^(3.0+-0.3)*R_e^(0.43+-0.19) or M_BH M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), similar to relations found observationally. The simulations indicate that the BHFP can be understood roughly as a tilted intrinsic correlation between BH mass and spheroid binding energy, or the condition for feedback coupling to power a pressure-driven outflow. While changes in halo circular velocity, merger orbital parameters, progenitor disk redshifts and gas fractions, ISM gas pressurization, and other parameters can drive changes in e.g. sigma at fixed M_bulge, and therefore changes in the M_BH-sigma or M_BH-M_bulge relations, the BHFP is robust. Given the empirical trend of decreasing R_e for a given M_bulge at high redshift, the BHFP predicts that BHs will be more massive at fixed M_bulge, in good agreement with recent observations. This evolution in the structural properties of merger remnants, to smaller R_e and larger sigma (and therefore larger M_BH, conserving the BHFP) at a given M_bulge, is driven by the fact that bulge progenitors have characteristically larger gas fractions at high redshifts. Adopting the observed evolution of disk gas fractions with redshift, our simulations predict the observed trends in both R_e(M_bulge) and M_BH(M_bulge).

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arXiv2007

An Observed Fundamental Plane Relation for Supermassive Black Holes

We study observed correlations between supermassive black hole (BHs) and the properties of their host galaxies, and show that the observations define a BH 'fundamental plane' (BHFP), of the form M_BH sigma^(3.0+-0.3)*R_e^(0.43+-0.19), or M_BH M_bulge^(0.54+-0.17)*sigma^(2.2+-0.5), analogous to the FP of elliptical galaxies. The BHFP is preferred over a simple relation between M_BH and any of sigma, M_bulge, M_dyn, or R_e alone at >99.9% significance. The existence of this BHFP has important implications for the formation of supermassive BHs and the masses of the very largest black holes, and immediately resolves several apparent conflicts between the BH masses expected and measured for outliers in both the M_BH-sigma and M_BH-M_bulge relations.

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arXiv2007

Photometric Properties of the Most Massive High-Redshift Galaxies

We calculate the observable properties of the most massive high-redshift galaxies in the hierarchical formation scenario where stellar spheroid and supermassive black hole growth are fueled by gas-rich mergers. Combining high-resolution hydrodynamical simulations of the hierarchical formation of a z~6 quasar, stellar population synthesis models, template AGN spectra, prescriptions for interstellar and intergalactic absorption, and the response of modern telescopes, the photometric evolution of galaxies destined to host z~6 quasars are modeled at redshifts z~4-14. These massive galaxies, with enormous stellar masses of M_star ~10^11.5-10^12 M_sun. and star formation rates of SFR~10^3-10^4 M_sun yr^-1 at z>~7, satisfy a variety of photometric selection criteria based on Lyman-break techniques including V-band dropouts at z>~5, i-band dropouts at z>~6, and z-band dropouts at z>~7. The observability of the most massive high-redshift galaxies is assessed and compared with a wide range of existing and future photometric surveys including SDSS, GOODS/HUDF, NOAO WDFS, UKIDSS, the IRAC Shallow Survey, Pan-STARRS, LSST, and SNAP. Massive stellar spheroids descended from z~6 quasars will likely be detected at z~4 by existing surveys, but owing to their low number densities the discovery of quasar progenitor galaxies at z>7 will likely require future surveys of large portions of the sky (>~0.5%) at wavelengths lambda>1 micron. The detection of rare, star-bursting, massive galaxies at redshifts z>~6 would provide support for the hierarchical formation of the earliest quasars and characterize the primitive star-formation histories of the most luminous elliptical galaxies.

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arXiv2007

Formation of z ~ 6 quasars from hierarchical galaxy mergers

The discovery of luminous quasars at redshift z ~ 6 indicates the presence of supermassive black holes (SMBHs) of mass ~10^9 Msun when the Universe was less than one billion years old. This finding presents several challenges for theoretical models. Here, we present the first multi-scale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6.5 in the LCDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~ 3 Gpc^3 volume, and find that this halo of ~ 8x 10^{12} Msun forming at z ~ 6.5 after several major mergers is able to reproduce a number of observed properties of SDSS J1148+5251, the most distant quasar detected at z =6.42 (Fan et al. 2003). Moreover, the SMBHs grow through gas accretion below the Eddington limit in a self-regulated manner owing to feedback. We find that the progenitors experience significant star formation (up to 10^4 Msun/yr) preceding the major quasar phase such that the stellar mass of the quasar host reaches 10^{12} Msun at z ~ 6.5, consistent with observations of significant metal enrichment in SDSS J1148+5251. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard LCDM cosmology, and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental SMBH-host correlation in a hierarchical Universe.(Abridged)

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arXiv2007

Observational Evidence for the Co-evolution of Galaxy Mergers, Quasars, and the Blue/Red Galaxy Transition

We compile a number of observations to estimate the time-averaged rate of formation or buildup of red sequence galaxies, as a function of mass and redshift. Comparing this with the mass functions of mergers and quasar hosts, and independently comparing their clustering properties as a function of redshift, we find that these populations trace the same mass distribution, with similar evolution, at redshifts 0 ~10^11 M_solar at z<~0.3) where 'dry' mergers appear to dominate. This supports a necessary prediction of theories where mergers between gas-rich galaxies produce ellipticals with an associated phase of quasar activity, after which the remnant becomes red. These populations trace a similar characteristic transition mass, possibly reflecting the mass above which the elliptical population is mostly (>~50%) assembled at a given redshift, which increases with redshift over the observed range in a manner consistent with suggestions that cosmic downsizing may apply to red galaxy assembly as well as star formation. These mass distributions as a function of redshift do not uniformly trace the all/red/blue galaxy population, ruling out models in which quasar activity is generically associated with star formation or is long lived in 'old' systems.

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arXiv2007

The Co-Formation of Spheroids and Quasars Traced in their Clustering

We compare observed clustering of quasars and galaxies as a function of redshift, mass, luminosity, & color/morphology, to constrain models of quasar fueling and spheroid-BH co-evolution. High redshift quasars are shown to be drawn from progenitors of local early-type galaxies, with the characteristic quasar luminosity L* reflecting a characteristic mass of 'active' BH/host populations at each redshift. Evolving observed high-z quasar clustering to z=0 predicts a trend of clustering in 'quasar remnants' as a function of stellar mass identical to that observed for early-types. However, quasar clustering does not simply reflect observed early (or late)-type populations; at each redshift, quasars cluster as an 'intermediate' population. Comparing with the age of elliptical stellar populations reveals that this 'intermediate' population represents those ellipticals undergoing or terminating their final significant star formation at each epoch. Assuming that quasar triggering is associated with the formation/termination epoch of ellipticals predicts quasar clustering at all observed redshifts without any model dependence or assumptions about quasar light curves, lifetimes, or accretion rates. This is not true for disks or quasar halos: i.e. quasars do not generically trace star formation, disks, or halo assembly. Quasar clustering at all z is consistent with a constant halo mass ~4x10^{12} M_sun, similar to local 'group scales.' The observations support a scenario in which major mergers trigger quasar activity and dominate bright, high-z quasar populations. We show that measurements of quasar clustering versus luminosity at z~1 can be used to constrain different lower-luminosity AGN fueling mechanisms, and that high-z clustering is sensitive to whether or not systems 'shut down' growth at z>3.

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arXiv2007

A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies: I. Galaxy Mergers & Quasar Activity

(Abridged) We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. Combining halo mass functions (MFs) with empirical halo occupation models, we calculate where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare with observed merger MFs, clustering, fractions, and small-scale environments, and show that this yields robust estimates in good agreement with observations. Making the simple ansatz that major, gas-rich mergers cause quasar activity, we demonstrate that this naturally reproduces the observed rise and fall of the quasar luminosity density from z=0-6, as well as quasar LFs, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The observed excess of quasar clustering on small scales is a natural prediction of the model, as mergers preferentially occur in regions with excess small-scale galaxy overdensities. We show that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where mergers of gas-rich galaxies are most efficient. We contrast with a secular model in which quasar activity is driven by bars/disk instabilities, and show that while these modes probably dominate at Seyfert luminosities, the constraints from clustering (large and small-scale), pseudobulge populations, disk MFs, luminosity density evolution, and host galaxy colors argue that they must be a small contributor to the z>1 quasar luminosity density.

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arXiv2007

A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes, and Elliptical Galaxies: II. Formation of Red Ellipticals

(Abridged) We develop and test a model for the cosmological role of mergers in the formation and quenching of red, early-type galaxies. Making the ansatz that star formation is quenched after a gas-rich, spheroid-forming major merger, we demonstrate that this naturally predicts the turnover in the efficiency of star formation at ~L_star, as well as the observed mass functions/density of red galaxies as a function of redshift, the formation times of spheroids as a function of mass, and the fraction of quenched galaxies as a function of galaxy and halo mass, environment, and redshift. Comparing to a variety of semi-analytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model and different broad classes of models make unique and robust qualitative predictions for a number of observables, including the red fraction as a function of galaxy and halo mass, the density of passive galaxies and evolution of the color-morphology-density relations at high z, and the fraction of disky/boxy spheroids as a function of mass. In each case, the observations favor a model in which galaxies quench after a major merger builds a massive spheroid, and disfavor quenching via secular or pure halo processes. We discuss a variety of physical possibilities for this quenching, and propose a mixed scenario in which traditional quenching in hot, massive halos is supplemented by the feedback associated with star formation and quasar activity in a major merger, which temporarily suppress cooling and establish the conditions of a dynamically hot halo in the central regions of the host, even in low mass halos.

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arXiv2007

Is the dependence of spectral index on luminosity real in optically selected AGN samples?

We critically examine the dependence of spectral index on luminosity in optically selected AGN samples. An analysis of optically selected high-redshift quasars showed an anti-correlation of $α_{OX}$, the spectral index between the rest-frame 2500 A and 2 keV, with optical luminosity (Miyaji et al. 2006). We examine this relationship by means of Monte Carlo simulations and conclude that a constant spectral index independent of optical luminosity is still consistent with this high-z sample. We further find that that contributions of large dispersions and narrow range of optical luminosity are most important for the apparent, yet artificial, $α_{OX} - l_o$ correlation reported. We also examine another, but more complete low-z optical selected AGN sub-sample from Steffen et al. (2006), and our analysis shows that a constant spectral index independent of optical luminosity is also consistent with the data. By comparing X-ray and optical luminosity functions, we find that a luminosity independent spectral index is in fact more preferred than the luminosity dependent spectral index model. We also discuss the selection effects caused by flux limits, which might systematically bias the $l_x - l_o$ relation and cause discrepancy in optically selected and X-ray selected AGN samples. To correctly establish a dependence of spectral index of AGNs on their luminosity, a larger and more complete sample is needed and consequences of luminosity dispersions and selection effects in flux limited samples must be taken into account properly.

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arXiv2007

The Shape, Multiplicity, and Evolution of Superclusters in LambdaCDM Cosmology

We determine the shape, multiplicity, size, and radial structure of superclusters in the LambdaCDM concordance cosmology from z = 0 to z = 2. Superclusters are defined as clusters of clusters in our large-scale cosmological simulation. We find that superclusters are triaxial in shape; many have flattened since early times to become nearly two-dimensional structures at present, with a small fraction of filamentary systems. The size and multiplicity functions are presented at different redshifts. Supercluster sizes extend to scales of ~ 100 - 200 Mpc/h. The supercluster multiplicity (richness) increases linearly with supercluster size. The density profile in superclusters is approximately isothermal (~ R^{-2}) and steepens on larger scales. These results can be used as a new test of the current cosmology when compared with upcoming observations of large-scale surveys.

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arXiv2007

Feedback-Driven Evolution of the Far-Infrared Spectral Energy Distributions of Luminous and Ultraluminous Infrared Galaxies

We calculate infrared spectral energy distributions (SEDs) from simulations of major galaxy mergers and study the effect of AGN and starburst driven feedback on the evolution of the SED as a function of time. We use a self-consistent three-dimensional radiative equilibrium code to calculate the emergent SEDs and to make images. To facilitate a simple description of our findings, we describe our results in reference to an approximate analytic solution for the far-IR SED. We focus mainly on the luminous infrared galaxy (LIRG) and ultraluminous infrared galaxy (ULIRG) phases of evolution. We contrast the SEDs of simulations performed with AGN feedback to simulations performed with starburst driven wind feedback. We find that the feedback processes critically determine the evolution of the SED. Changing the source of illumination (whether stellar or AGN) has virtually no impact on the reprocessed far-infrared SED. We find that AGN feedback is particularly effective at dispersing gas and rapidly injecting energy into the ISM. The observational signature of such powerful feedback is a warm SED. In general, simulations performed with starburst driven winds have colder spectra and reprocess more of their emission into the infrared, resulting in higher infrared to bolometric luminosities compared to (otherwise equivalent) simulations performed with AGN feedback. We depict our results in IRAS bands, as well as in Spitzer's MIPS bands, and in Herschel's PACS bands.

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