We develop a model for the cosmological role of mergers in the evolution of starbursts, quasars, and spheroidal galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we can estimate where galaxies of given properties live at a particular epoch. This allows us to calculate, in an a priori cosmological manner, where major galaxy-galaxy mergers occur and what kinds of galaxies merge, at all redshifts. We compare this with the observed mass functions, clustering, fractions as a function of halo and galaxy mass, and small-scale environments of mergers, and show that this approach yields robust estimates in good agreement with observations, and can be extended to predict detailed properties of mergers. Making the simple ansatz that major, gas-rich mergers cause quasar activity (but not strictly assuming they are the only triggering mechanism), we demonstrate that this model naturally reproduces the observed rise and fall of the quasar luminosity density from z = 0 − 6, as well as quasar luminosity functions, fractions, host galaxy colors, and clustering as a function of redshift and luminosity. The recent observed excess of quasar clustering on small scales at z ∼ 0.2 − 2.5 is a natural prediction of our model, as mergers will preferentially occur in regions with excess small-scale galaxy overdensities. In fact, we demonstrate that quasar environments at all observed redshifts correspond closely to the empirically determined small group scale, where major mergers of ∼ L * gas-rich galaxies will be most efficient. We contrast this with a secular model in which quasar activity is driven by bars or other disk instabilities, and show that while these modes of fueling probably dominate the high-Eddington ratio population at Seyfert luminosities (significant at z = 0), the constraints from quasar clustering, observed pseudobulge populations, and disk mass functions suggest that they are a small contributor to the z 1 quasar luminosity density, which is dominated by massive BHs in predominantly classical spheroids formed in mergers. Similarly, lowluminosity Seyferts do not show a clustering excess on small scales, in agreement with the natural prediction of secular models, but bright quasars at all redshifts do so. We also compare recent observations of the colors of quasar host galaxies, and show that these correspond to the colors of recent merger remnants, in the transition region between the blue cloud and the red sequence, and are distinct from the colors of systems with observed bars or strong disk instabilities. Even the most extreme secular models, in which all bulge (and therefore BH) formation proceeds via disk instability, are forced to assume that this instability acts before the (dynamically inevitable) mergers, and therefore predict a history for the quasar luminosity density which is shifted to earlier times, in disagreement with observations. Our model provides a powerful means to predict the abunda...

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 Lambda cold dark matter (ΛCDM) 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 cut‐off 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 and black holes 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’ΛCDM 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).

We develop a general physical model for how galactic disks survive and/or are destroyed in mergers and interactions. Based on simple dynamical arguments, we show that gas primarily loses angular momentum to internal torques in a merger, induced by the gravity of the secondary. Gas within some characteristic radius, determined by the efficiency of this angular momentum loss (itself a function of the orbital parameters, mass ratio, and gas fraction of the merging galaxies), will quickly lose angular momentum to the stars sharing the perturbed host disk, fall to the center and be consumed in a starburst. We use a similar analysis to determine where violent relaxation of the pre-merger stellar disks is efficient on final coalescence. Our model describes both the dissipational and dissipationless components of the merger, and allows us to predict, for a given arbitrary encounter, the stellar and gas content of the material that will survive (without significant angular momentum loss or violent relaxation) to re-form a disk in the merger remnant, versus being dissipationlessly violently relaxed or dissipationally losing angular momentum and forming a compact central starburst. We test these predictions with a large library of hydrodynamic merger simulations, and show that they agree well (with small scatter) with the properties of simulated merger remnants as a function of merger mass ratio, orbital parameters, and gas distributions, in simulations which span a wide range of parameter space in these properties as well as prescriptions for gas physics, stellar and AGN feedback, halo and initial disk structural properties, redshift, and galaxy masses. We show that, in an immediate (short-term) sense, the amount of stellar or gaseous disk that survives or re-forms following a given interaction can be understood purely in terms of simple, well-understood gravitational physics, independent of the details of the ISM gas physics or stellar and AGN feedback. This allows us to demonstrate and quantify how these physics are in fact important, in an indirect sense, to enable disks to survive mergers, by lowering star formation efficiencies in low mass systems (allowing them to retain large gas fractions) and distributing the gas to large radii. The efficiency of disk destruction in mergers is a strong function of gas content -our model allows us to explicitly predict and demonstrate how, in sufficiently gas rich mergers (with quite general orbital parameters), even 1:1 mass-ratio mergers can yield disk-dominated remnants, and more realistic 1:3-1:4 mass-ratio major mergers can yield systems with < 20% of their mass in bulges. We discuss a number of implications of this modeling for the abundance and morphology of bulges as a function of mass and redshift, and provide simple prescriptions for the implementation of our results in analytic or semi-analytic models of galaxy formation.

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 disc 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 parametrization. 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 disc and suppresses merger-driven star formation for large mass ratio mergers. Direct, coplanar merging orbits produce the largest tidal disturbance and yield the 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 smoothed particle hydrodynamics employed here, the burst efficiencies of the mergers presented here are smaller than in previous studies.

Recent proper motion measurements of the Large and Small Magellanic Clouds (LMC and SMC, respectively) by Kallivayalil et al. (2006a,b) suggest that the 3D velocities of the Clouds are substantially higher (∼100 km/s) than previously estimated and now approach the escape velocity of the Milky Way (MW). Previous studies have also assumed that the Milky Way can be adequately modeled as an isothermal sphere to large distances. Here we re-examine the orbital history of the Clouds using the new velocities and a ΛCDM-motivated MW model with virial mass M vir = 10 12 M ⊙ (e.g. Klypin et al. (2002)). We conclude that the L/SMC are either currently on their first passage about the MW or, if the MW can be accurately modeled by an isothermal sphere to distances 200 kpc (i.e., M vir > 2 × 10 12 M ⊙ ), that their orbital period and apogalacticon distance must be a factor of two larger than previously estimated, increasing to 3 Gyr and 200 kpc, respectively. A first passage scenario is consistent with the fact that the LMC and SMC appear to be outliers when compared to other satellite galaxies of the MW: they are irregular in appearance and are moving faster. We discuss the implications of this orbital analysis for our understanding of the star formation history, the nature of the warp in the MW disk and the origin of the Magellanic Stream (MS), a band of HI gas trailing the LMC and SMC that extends ∼100 degrees across the sky. Specifically, as a consequence of the new orbital history of the Clouds, the origin of the MS may not be explainable by current tidal and ram pressure stripping models.

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 − M 20 , 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 comoving volume for samples selected with constant number density evolves much more strongly with redshift (∝ (1 + z) +3.0±1.1 ) than samples selected with constant stellar mass or passively evolving luminosity (∝ (1 + z) +0.1±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 − M 20 . The implied minor merger rate is ∼ 3 times the major merger rate at z ∼ 0.7, and shows little evolution with redshift.

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 gadgetN‐body/hydrodynamical 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.

The violent hierarchical nature of the Λ-Cold Dark Matter cosmology poses serious difficulties for the formation of disk galaxies. To help resolve these issues, we describe a new, merger-driven scenario for the cosmological formation of disk galaxies at high redshifts that supplements the standard model based on dissipational collapse. In this picture, large gaseous disks may be produced from highangular momentum mergers of systems that are gas-dominated, i.e. M gas /(M gas + M ⋆ ) 0.5 at the height of the merger. Pressurization from the multiphase structure of the interstellar medium prevents the complete conversion of gas into stars during the merger, and if enough gas remains to form a disk, the remnant eventually resembles a disk galaxy. We perform numerical simulations of galaxy mergers to study how supernovae feedback strength, supermassive black hole growth and feedback, progenitor gas fraction, merger mass-ratio, and orbital geometry impact the formation of remnant disks. We find that disks can build angular momentum through mergers and the degree of rotational support of the baryons in the merger remnant is primarily related to feedback processes associated with star formation. Nearly every simulated gas-rich merger remnant contains rapidlyrotating stellar substructure, while disk-dominated remnants are restricted to form in mergers that are gas-dominated at the time of final coalescence. Typically, gas-dominated mergers require extreme progenitor gas fractions (f gas > 0.8). We also show that the formation of rotationally-supported stellar systems in mergers is not restricted to idealized orbits, and both gas-rich major and minor mergers can produce disk-dominated stellar remnants. We suggest that the hierarchical nature of the Λ-Cold Dark Matter cosmology and the physics of the interstellar gas may act together to form spiral galaxies by building the angular momentum of disks through early, gas-dominated mergers. Our proposed scenario may be especially important for galaxy formation at high redshifts, where gas-dominated mergers are believed to be more common than in the local Universe.