The Relation Between Quasar and Merging Galaxy Luminosity Functions and the Merger-Induced Star Formation Rate of the Universe

Using a model for self-regulated growth of black holes (BHs) in mergers involving gas-rich galaxies, we study the relationship between quasars and the population of merging galaxies and predict the merger-induced star formation rate density of the Universe. Mergers drive nuclear gas inflows, fueling starbursts and 'buried quasars' until accretion feedback expels the gas, rendering a briefly visible optical quasar. Star formation is shut down and accretion declines, leaving a passively evolving remnant with properties typical of red, elliptical galaxies. Based on evolution of these events in our simulations, we demonstrate that the observed statistics of merger rates, luminosity functions (LFs) and mass functions, SFR distributions, specific SFRs, quasar and quasar host galaxy LFs, and elliptical/red galaxy LFs are self-consistent and follow from one another as predicted by the merger hypothesis. We use our simulations to de-convolve both quasar and merging galaxy LFs to determine the birthrate of black holes of a given final mass and merger rates as a function of stellar mass. We use this to predict the merging galaxy LF in several observed wavebands, color-magnitude relations, mass functions, absolute and specific SFR distributions and SFR density, and quasar host galaxy LFs, as a function of redshift from z=0-6. We invert this and predict e.g. quasar LFs from observed merger LFs or SFR distributions. Our results agree well with observations, but idealized models of quasar lightcurves are ruled out by comparison of merger and quasar observations at >99.9% confidence. Using only observations of quasars, we estimate the contribution of mergers to the SFR density of the Universe even to high redshifts z~4.Comment: 26 pages, 15 figures, matches version accepted to Ap

A Unified Model for the Evolution of Galaxies and Quasars

We incorporate a simple scheme for the growth of supermassive black holes into semi-analytic models that follow the formation and evolution of galaxies in a cold dark matter dominated Universe. We assume that supermassive black holes are formed and fuelled during major mergers. If two galaxies of comparable mass merge, their central black holes coalesce and a few percent of the gas in the merger remnant is accreted by the new black hole over a timescale of a few times 10^7 years. With these simple assumptions, our model not only fits many aspects of the observed evolution of galaxies, but also reproduces quantitatively the observed relation between bulge luminosity and black hole mass in nearby galaxies, the strong evolution of the quasar population with redshift and the relation between the luminosities of nearby quasars and those of their host galaxies. The strong decline in the number density of quasars from z=2 to z=0 is due to the combination of three effects: i) a decrease in the merging rate, ii) a decrease in the amount of cold gas available to fuel black holes, and iii) an increase in the timescale for gas accretion. In a LCDM cosmology the predicted decline in the total content of cold gas in galaxies is consistent with that inferred from observations of damped Lyman-alpha systems. Our results strongly suggest that the evolution of supermassive black holes, quasars and starbursts is inextricably linked to the hierarchical build-up of galaxies.Comment: 30 pages, Latex, 18 figures included, submitted to MNRA

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.Comment: 34 pages, 27 figures, submitted to ApJ. Fixed appearance of Figure

Mergers, AGN, and 'Normal' Galaxies: Contributions to the Distribution of Star Formation Rates and Infrared Luminosity Functions

We use a novel method to predict the contribution of normal star-forming galaxies, merger-induced bursts, and obscured AGN, to IR luminosity functions (LFs) and global SFR densities. We use empirical halo occupation constraints to populate halos with galaxies and determine the distribution of normal and merging galaxies. Each system can then be associated with high-resolution hydrodynamic simulations. We predict the distribution of observed luminosities and SFRs, from different galaxy classes, as a function of redshift from z=0-6. We provide fitting functions for the predicted LFs, quantify the uncertainties, and compare with observations. At all redshifts, 'normal' galaxies dominate the LF at moderate luminosities ~L* (the 'knee'). Merger-induced bursts increasingly dominate at L>>L*; at the most extreme luminosities, AGN are important. However, all populations increase in luminosity at higher redshifts, owing to increasing gas fractions. Thus the 'transition' between normal and merger-dominated sources increases from the LIRG-ULIRG threshold at z~0 to bright Hyper-LIRG thresholds at z~2. The transition to dominance by obscured AGN evolves similarly, at factor of several higher L_IR. At all redshifts, non-merging systems dominate the total luminosity/SFR density, with merger-induced bursts constituting ~5-10% and AGN ~1-5%. Bursts contribute little to scatter in the SFR-stellar mass relation. In fact, many systems identified as 'ongoing' mergers will be forming stars in their 'normal' (non-burst) mode. Counting this as 'merger-induced' star formation leads to a stronger apparent redshift evolution in the contribution of mergers to the SFR density.Comment: 16 pages, 9 figures (+appendices), accepted to MNRAS. A routine to return the galaxy merger rates discussed here is available at http://www.cfa.harvard.edu/~phopkins/Site/mergercalc.htm

Determining the Properties and Evolution of Red Galaxies from the Quasar Luminosity Function

(Abridged) We study the link between quasars and the red galaxy population using a model for the self-regulated growth of supermassive black holes in mergers involving gas-rich galaxies. Using a model for quasar lifetimes and evolution motivated by hydrodynamical simulations of galaxy mergers, we de-convolve the observed quasar luminosity function at various redshifts to determine the rate of formation of black holes of a given final mass. Identifying quasar activity with the formation of spheroids in the framework of the merger hypothesis, this enables us to deduce the corresponding rate of formation of spheroids with given properties as a function of redshift. This allows us to predict, for the red galaxy population, the distribution of galaxy velocity dispersions, the mass function, mass density, star formation rates, the luminosity function in many observed wavebands (NUV, U, B, V, R, I, J, H, K), the total red galaxy number density and luminosity density, the distribution of colors as a function of magnitude and velocity dispersion for several different wavebands, the distribution of mass to light ratios vs. mass, the luminosity-size relations, and the typical ages and distribution of ages (formation redshifts) as a function of both mass and luminosity. For each of these quantities, we predict the evolution from redshift z=0-6. Each of our predictions agrees well with existing observations, without the addition of tunable parameters; the essential observational inputs come from the observed quasar luminosity function. These predictions are skewed by several orders of magnitude if we adopt simpler, traditional models of quasar lifetimes in which quasars turn on/off or follow simple exponential light curves, instead of the more complicated evolution implied by our simulations.Comment: 28 pages, 22 figures, matches version accepted to Ap

GECO: Galaxy Evolution COde - A new semi-analytical model of galaxy formation

We present a new semi-analytical model of galaxy formation, GECO (Galaxy Evolution COde), aimed at a better understanding of when and how the two processes of star formation and galaxy assembly have taken place. Our model is structured into a Monte Carlo algorithm based on the Extended Press-Schechter theory, for the representation of the merging hierarchy of dark matter halos, and a set of analytic algorithms for the treatment of the baryonic physics, including classical recipes for the gas cooling, the star formation time-scales, galaxy mergers and SN feedback. Together with the galaxies, the parallel growth of BHs is followed in time and their feedback on the hosting galaxies is modelled. We set the model free parameters by matching with data on local stellar mass functions and the BH-bulge relation at z=0. Based on such local boundary conditions, we investigate how data on the high-redshift universe constrain our understanding of the physical processes driving the evolution, focusing in particular on the assembly of stellar mass and on the star formation history. Since both processes are currently strongly constrained by cosmological near- and far-IR surveys, the basic physics of the Lambda CDM hierarchical clustering concept of galaxy formation can be effectively tested by us by comparison with the most reliable set of observables. Our investigation shows that when the time-scales of the stellar formation and mass assembly are studied as a function of dark matter halo mass and the single galaxy stellar mass, the 'downsizing' fashion of star formation appears to be a natural outcome of the model, reproduced even in the absence of the AGN feedback. On the contrary, the stellar mass assembly history turns out to follow a more standard hierarchical pattern progressive in cosmic time, with the more massive systems assembled at late times mainly through dissipationless mergers.Comment: Accepted for publication in A&A, 24 pages, 15 figure

Strongly star-forming rotating disks in a complex merging system at z = 4,7 as revealed by ALMA

We performed a kinematical analysis of the [CII] line emission of the BR 1202-0725 system at z~4,7 using ALMA observations. The most prominent sources of this system are a quasar and a submillimeter galaxy, separated by a projected distance of about 24 kpc and characterized by very high SFR, higher than 1000 Msun/yr. However, the ALMA observations reveal that these galaxies apparently have undisturbed rotating disks, which is at variance with the commonly accepted scenario in which strong star formation activity is induced by a major merger. We also detected faint components which, after spectral deblending, were spatially resolved from the main QSO and SMG emissions. The relative velocities and positions of these components are compatible with orbital motions within the gravitational potentials generated by the QSO host galaxy and the SMG, suggesting that they are smaller galaxies in interaction or gas clouds in accretion flows of tidal streams. We did not find any clear spectral evidence for outflows caused by AGN or stellar feedback. This suggests that the high star formation rates might be induced by interactions or minor mergers with these companions, which do not affect the large-scale kinematics of the disks, however. Our kinematical analysis also indicates that the QSO and the SMG have similar Mdyn, mostly in the form of molecular gas, and that the QSO host galaxy and the SMG are seen close to face-on with slightly different disk inclinations: the QSO host galaxy is seen almost face-on (i~15), while the SMG is seen at higher inclinations (i~25). Finally, the ratio between the black hole mass of the QSO, obtained from XShooter spectroscopy, and the Mdyn of the host galaxy is similar to value found in very massive local galaxies, suggesting that the evolution of black hole galaxy relations is probably better studied with dynamical than with stellar host galaxy masses.Comment: Accepted for publication in Astronomy and Astrophysic

The Demography of Super-Massive Black Holes: Growing Monsters at the Heart of Galaxies

Supermassive black holes (BHs) appear to be ubiquitous at the center of all galaxies which have been observed at high enough sensitivities and resolution with the Hubble Space Telescope. Their masses are found to be tightly linked with the masses and velocity dispersions of their host galaxies. On the other hand, BHs are widely held to constitute the central engines of quasars and active galactic nuclei (AGN) in general. It is however still unclear how BHs have grown, and whether they have co-evolved with their hosts. In this Review I discuss how, in ways independent of specific models, constraints on the growth history of BHs and their host galaxies have been set by matching the statistics of local BHs to the emissivity, number density, and clustering properties of AGNs at different cosmological epochs. I also present some new results obtained through a novel numerical code which evolves the BH mass function and clustering adopting broad distributions of Eddington ratios. I finally review BH evolution in a wider cosmological context, connecting BH growth to galaxy evolution.Comment: 70 pages. New Astronomy Reviews, in pres

The Formation of the First Massive Black Holes

Supermassive black holes (SMBHs) are common in local galactic nuclei, and SMBHs as massive as several billion solar masses already exist at redshift z=6. These earliest SMBHs may grow by the combination of radiation-pressure-limited accretion and mergers of stellar-mass seed BHs, left behind by the first generation of metal-free stars, or may be formed by more rapid direct collapse of gas in rare special environments where dense gas can accumulate without first fragmenting into stars. This chapter offers a review of these two competing scenarios, as well as some more exotic alternative ideas. It also briefly discusses how the different models may be distinguished in the future by observations with JWST, (e)LISA and other instruments.Comment: 47 pages with 306 references; this review is a chapter in "The First Galaxies - Theoretical Predictions and Observational Clues", Springer Astrophysics and Space Science Library, Eds. T. Wiklind, V. Bromm & B. Mobasher, in pres

Do We Expect Most AGN to Live in Disks?

Recent observations have indicated that a large fraction of the low to intermediate luminosity AGN population lives in disk-dominated hosts, while the more luminous quasars live in bulge-dominated hosts, in conflict with some previous model predictions. We therefore build and compare a semi-empirical model for AGN fueling which accounts for both merger and non-merger 'triggering.' In particular, we show that the 'stochastic accretion' model - in which fueling in disk galaxies is essentially a random process arising whenever dense gas clouds reach the nucleus - provides a good match to the present observations at low/intermediate luminosities. However it falls short of the high-luminosity population. We combine this with models for major merger-induced AGN fueling, which lead to rarer but more luminous events, and predict the resulting abundance of disk-dominated and bulge-dominated AGN host galaxies as a function of luminosity and redshift. We compile and compare observational constraints from z~0-2. The models and observations generically show a transition from disk to bulge dominance in hosts near the Seyfert-quasar transition, at all redshifts. 'Stochastic' fueling dominates AGN by number (dominant at low luminosity), and dominates BH growth below the knee in the present-day BH mass function (<10^7 M_sun). However it accounts for just ~10% of BH mass growth at masses >10^8 M_sun. In total, fueling in disky hosts accounts for ~30% of the total AGN luminosity density/BH mass density. The combined model also accurately predicts the AGN luminosity function and clustering/bias as a function of luminosity and redshift; however, we argue that these are not sensitive probes of BH fueling mechanisms.Comment: 13 pages, 5 figures, PDF updated to match published versio