Dušan Kereš

How do Galaxies Get Their Gas

Not the way one might have thought. In hydrodynamic simulations of galaxy formation, some gas follows the traditionally envisioned route, shock heating to the halo virial temperature before cooling to the much lower temperature of the neutral ISM. But most gas enters galaxies without ever heating close to the virial temperature, gaining thermal energy from weak shocks and adiabatic compression, and radiating it just as quickly. This “cold mode” accretion is channeled along filaments, while the conventional, “hot mode” accretion is quasi-spherical. Cold mode accretion dominates high redshift growth by a substantial factor, while at z < 1 the overall accretion rate declines and hot mode accretion has greater relative importance. The decline of the cosmic star formation rate at low z is driven largely by geometry, as the typical cross section of filaments begins to exceed that of the galaxies at their intersections.

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

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 we 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 at -->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 we 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, low-luminosity 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 that is shifted to earlier times, in disagreement with observations. Our model provides a powerful means to predict the abundance and nature of mergers and to contrast cosmologically motivated predictions of merger products such as starbursts and active galactic nuclei.

Galaxies in a simulated ΛCDM Universe – I. Cold mode and hot cores

We study the formation of galaxies in a large volume (50 h −1 Mpc, 2 × 288 3 particles) cosmological simulation, evolved using the entropy and energy-conserving smoothed particle hydrodynamics (SPH) code GADGET-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary ‘cold mode’ accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code. Atmospheres of hot, virialized gas develop in haloes above 2–3 × 10 11 M � , a transition mass that is nearly constant from z = 3 to 0. Cold accretion persists in haloes above the transition mass, especially at z ≥ 2. It dominates the growth of galaxies in low-mass haloes at all times, and it is the main driver of the cosmic star formation history. Our results suggest that the cooling of shock-heated virialized gas, which has been the focus of many analytic models of galaxy growth spanning more than three decades, might be a relatively minor element of galaxy formation. At high redshifts, satellite galaxies have gas accretion rates similar to central galaxies of the same baryonic mass, but at z < 1t he accretion rates of low-mass satellites are well below those of comparable central galaxies. Relative to our earlier simulations, the GADGET-2 simulations predict much lower rates of ‘hot mode’ accretion from the virialized gas component. Hot accretion rates compete with cold accretion rates near the transition mass, but only at z ≤ 1. Hot accretion is inefficient in haloes

Feedback and recycled wind accretion: assembling the z= 0 galaxy mass function

We analyse cosmological hydrodynamic simulations that include theoretically and observationally motivated prescriptions for galactic outflows. If these simulated winds accurately represent winds in the real Universe, then material previously ejected in winds provides the dominant source of gas infall for new star formation at redshifts z < 1. This recycled wind accretion, or wind mode, provides a third physically distinct accretion channel in addition to the hot and cold modes emphasized in recent theoretical studies. The recycling time of wind material (t rec ) is shorter in higher mass systems owing to the interaction between outflows and the increasingly higher gas densities in and around higher mass haloes. This differential recycling plays a central role in shaping the present-day galaxy stellar mass function (GSMF), because declining t rec leads to increasing wind mode galaxy growth in more massive haloes. For the three feedback models explored, the wind mode dominates above a threshold mass that primarily depends on wind velocity; the shape of the GSMF therefore can be directly traced back to the feedback prescription used. If we remove all particles that were ever ejected in a wind, then the predicted GSMFs are much steeper than observed. In this case, galaxy masses are suppressed both by the ejection of gas from galaxies and by the hydrodynamic heating of their surroundings, which reduces subsequent infall. With wind recycling included, the simulation that incorporates our favoured momentum-driven wind scalings reproduces the observed GSMF for stellar masses 10 9 M ⊙ M ≤ 5 × 10 10 M ⊙ . At higher masses, wind recycling leads to excessive galaxy masses and star formation rates relative to observations. In these massive systems, some quenching mechanism must suppress not only the direct accretion from the primordial intergalactic medium but the re-accretion of gas ejected from star-forming galaxies. In short, as has long been anticipated, the form of the GSMF is governed by outflows; the unexpected twist here for our simulated winds is that it is not primarily the ejection of material but how the ejected material is re-accreted that governs the GSMF.

MERGERS AND BULGE FORMATION IN ΛCDM: WHICH MERGERS MATTER?

We use a suite of semi-empirical models to predict the galaxy-galaxy merger rate and relative contributions to bulge growth as a function of mass (both halo and stellar), redshift, and mass ratio. The models use empirical constraints on the halo occupation distribution, evolved forward in time, to robustly identify where and when galaxy mergers occur. Together with the results of high-resolution merger simulations, this allows us to quantify the relative contributions of mergers with different properties (e.g., mass ratios, gas fractions, redshifts) to the bulge population. We compare with observational constraints, and find good agreement. We also provide useful fitting functions and make public a code to reproduce the predicted merger rates and contributions to bulge mass growth. We identify several robust conclusions. (1) Major mergers dominate the formation and assembly of ~L * bulges and the total spheroid mass density, but minor mergers contribute a non-negligible ~30%. (2) This is mass dependent: bulge formation and assembly is dominated by more minor mergers in lower-mass systems. In higher-mass systems, most bulges originally form in major mergers near ~L *, but assemble in increasingly minor mergers. (3) The minor/major contribution is also morphology dependent: higher B/T systems preferentially form in more major mergers, with B/T roughly tracing the mass ratio of the largest recent merger; lower B/T systems preferentially form in situ from minor mergers. (4) Low-mass galaxies, being gas-rich, require more mergers to reach the same B/T as high-mass systems. Gas-richness dramatically suppresses the absolute efficiency of bulge formation, but does not strongly influence the relative contribution of major versus minor mergers. (5) Absolute merger rates at fixed mass ratio increase with galaxy mass. (6) Predicted merger rates agree well with those observed in pair and morphology-selected samples, but there is evidence that some morphology-selected samples include contamination from minor mergers. (7) Predicted rates also agree with the integrated growth in bulge mass density with cosmic time, but with a factor ~2 uncertainty in both—up to half the bulge mass density could come from non-merger processes. We systematically vary the model assumptions, totaling ~103 model permutations, and quantify the resulting uncertainties. Our conclusions regarding the importance of different mergers for bulge formation are very robust to these changes. The absolute predicted merger rates are systematically uncertain at the factor ~2 level; uncertainties grow at the lowest masses and high redshifts.

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

We develop and test a model for the cosmological role of mergers in the formation and quenching of red, early-type galaxies. By combining theoretically well-constrained halo and subhalo mass functions as a function of redshift and environment with empirical halo occupation models, we predict the distribution of mergers as a function of redshift, environment, and physical galaxy properties. Making the simple 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 and baryon fractions in galaxies at ~ -->L* (without any parameters tuned to this value), as well as the observed mass functions and mass density of red galaxies as a function of redshift, the formation times of early-type galaxies as a function of mass, and the fraction of quenched galaxies as a function of galaxy and halo mass, environment, and redshift. Comparing our model to a variety of semianalytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model makes unique and robust qualitative predictions for a number of observables, including the bivariate red fraction as a function of galaxy and halo mass, the density of passive galaxies at high redshifts, the emergence/evolution of the color-morphology-density relations at high redshift, and the fraction of disky/boxy (or cusp/core) spheroids as a function of mass. In each case, the observations favor a model in which some mechanism quenches future star formation after a major merger builds a massive spheroid. Models where quenching is dominated by a halo mass threshold fail to match the behavior of the bivariate red fractions, predict too low a density of passive galaxies at high redshift, and overpredict by an order of magnitude the mass of the transition from disky to boxy ellipticals. Models driven by secular disk instabilities also qualitatively disagree with the bivariate red fractions, fail to predict the observed evolution in the color-density relations, and predict order-of-magnitude incorrect distributions of kinematic types in early-type galaxies. We make specific predictions for how future observations, for example, quantifying the red fraction as a function of galaxy mass, halo mass, environment, or redshift, can break the degeneracies between a number of different assumptions adopted in present galaxy formation models. We discuss a variety of physical possibilities for this quenching and propose a mixed scenario in which traditional quenching in hot, quasi-static massive halos is supplemented by the strong shocks and feedback energy input associated with a major merger (e.g., tidal shocks, starburst-driven winds, and quasar feedback), 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 (below the traditional threshold for accretion shocks).

DISSIPATION AND EXTRA LIGHT IN GALACTIC NUCLEI. IV. EVOLUTION IN THE SCALING RELATIONS OF SPHEROIDS

We develop a model for the physical origin and redshift evolution of spheroid scaling relations. We consider spheroid sizes, velocity dispersions, dynamical masses, profile shapes (Sersic indices), stellar and supermassive black hole (BH) masses, and their related scalings. Our approach combines advantages of prior observational constraints in halo occupation models and libraries of high-resolution hydrodynamic simulations of galaxy mergers. This allows us to separate the relative roles of dissipation, dry mergers, formation time, and evolution in progenitor properties, and identify their impact on observed scalings at each redshift. We show that, at all redshifts, dissipation is the most important factor determining spheroid sizes and fundamental plane scalings, and can account (at z = 0) for the observed fundamental plane tilt and differences between observed disk and spheroid scaling relations. Because disks (spheroid progenitors) at high redshift have characteristically larger gas fractions, this predicts more dissipation in mergers, yielding systematically more compact, smaller spheroids. In detail, this gives rise to a mass-dependent evolution in the sizes of spheroids of a given mass, which agrees well with observations. This relates to a subtle weakening of the tilt of the early-type fundamental plane with redshift, important for a number of studies that assume a nonevolving stellar mass fundamental plane. This also predicts evolution in the BH-host mass relations, toward more massive BHs at higher redshifts. Dry mergers are also significant, but only for large systems which form early—they originate as compact systems, but undergo a number of dry mergers (consistent with observations) such that they have sizes at any later observed redshift similar to systems of the same mass formed more recently. Most of the observed, compact high-redshift ellipticals will become the cores of present brightest cluster galaxies, and we show how their sizes, velocity dispersions, and BH masses evolve to become consistent with observations. We also predict what fraction might survive intact from early formation and identify their characteristic z = 0 properties. We make predictions for residual correlations as well, e.g., the correlation of size and fundamental plane residuals with formation time of a given elliptical, that can be used as additional tests of these models.

The baryonic assembly of dark matter haloes

We use a suite of cosmological hydrodynamic simulations to systematically quantify the accretion rates of baryons into dark matter haloes and the resulting baryon mass fractions, as a function of halo mass, redshift and baryon type (including cold and hot gas). We find that the net baryonic accretion rates through the virial radius are sensitive to galactic outflows and explore a range of outflow parameters to illustrate the effects. We show that the cold gas accretion rate is in general not a simple universal factor of the dark matter accretion rate, and that galactic winds can cause star formation rates to deviate significantly from the external gas accretion rates, both via gas ejection and re-accretion. Furthermore, galactic winds can inject enough energy and momentum in the surrounding medium to slow down accretion altogether, especially in low-mass haloes and at low redshift, but the impact of outflows is suppressed with increasing halo mass. By resolving the accretion rates versus radius from the halo centres, we show how cold streams penetrate the hot atmospheres of massive haloes at z≥ 2, but gradually disappear at lower redshift. The total baryon mass fraction is also strongly suppressed by outflows in low-mass haloes, but is nearly universal in the absence of feedback in haloes above the UV background suppression scale, corresponding to circular velocities vc∼ 50xa0kmxa0s−1. The transition halo mass, at which the gas mass in haloes is equal for the cold and hot components, is roughly constant at ∼1011.5xa0M⊙ and does not depend sensitively on the wind prescription. We provide simple fitting formulae for the cold gas accretion rate and the corresponding efficiency with which dark matter channels cold gas into haloes in the no-wind case. Finally, we show that cold accretion is broadly consistent with driving the bulk of the highly star-forming galaxies observed at z∼ 2, but that the more intense star formers likely sample the high end of the accretion rate distribution, and may be additionally fuelled by a combination of gas recycling, gas re-accretion, hot mode cooling and mergers.

The nature of submillimetre galaxies in cosmological hydrodynamic simulations

We study the nature of rapidly star-forming galaxies at z= 2 in cosmological hydrodynamic simulations, and compare their properties to observations of submillimetre galaxies (SMGs). We identify simulated SMGs as the most rapidly star-forming systems that match the observed number density of SMGs. In our models, SMGs are massive galaxies sitting at the centres of large potential wells, being fed by smooth infall and gas-rich satellites at rates comparable to their star formation rates (SFRs). They are not typically undergoing major mergers that significantly boost their quiescent SFR, but they still often show complex gas morphologies and kinematics. Our simulated SMGs have stellar masses of M*∼ 1011−11.7xa0M⊙, SFRs of ∼180–500xa0M⊙xa0yr−1, a clustering length of ∼10xa0h−1xa0Mpc and solar metallicities. The SFRs are lower than those inferred from far-infrared (far-IR) data by ∼×3, which we suggest may owe to one or more systematic effects in the SFR calibrations. SMGs at z= 2 live in ∼1013xa0M⊙ haloes, and by z= 0 they mostly end up as brightest group galaxies in ∼1014xa0M⊙ haloes. We predict that higher M* SMGs should have on average lower specific SFRs, less disturbed morphologies and higher clustering. We also predict that deeper far-IR surveys will smoothly join SMGs on to the massive end of the SFR–M* relationship defined by lower mass z∼ 2 galaxies. Overall, our simulated rapid star-formers provide as good a match to available SMG data as merger-based scenarios, offering an alternative scenario that emerges naturally from cosmological simulations.

Mergers in λCDM: Uncertainties in theoretical predictions and interpretations of the merger rate

Different theoretical methodologies lead to order-of-magnitude variations in predicted galaxy-galaxy merger rates. We examine how this arises and quantify the dominant uncertainties. Modeling of dark matter and galaxy inspiral/merger times contribute factor of {approx}2 uncertainties. Different estimates of the halo-halo merger rate, the subhalo destruction rate, and the halo merger rate with some dynamical friction time delay for galaxy-galaxy mergers, agree to within this factor of {approx}2, provided proper care is taken to define mergers consistently. There are some caveats: if halo/subhalo masses are not appropriately defined the major-merger rate can be dramatically suppressed, and in models with orphan galaxies and under-resolved subhalos the merger timescale can be severely over-estimated. The dominant differences in galaxy-galaxy merger rates between models owe to the treatment of the baryonic physics. Cosmological hydrodynamic simulations without strong feedback and some older semi-analytic models (SAMs), with known discrepancies in mass functions, can be biased by large factors ({approx}5) in predicted merger rates. However, provided that models yield a reasonable match to the total galaxy mass function, the differences in properties of central galaxies are sufficiently small to alone contribute small (factor of {approx}1.5) additional systematics to merger rate predictions. But variations in the baryonic physics ofmorexa0» satellite galaxies in models can also have a dramatic effect on merger rates. The well-known problem of satellite over-quenching in most current SAMs-whereby SAM satellite populations are too efficiently stripped of their gas-could lead to order-of-magnitude under-estimates of merger rates for low-mass, gas-rich galaxies. Models in which the masses of satellites are fixed by observations (or SAMs adjusted to resolve this over-quenching) tend to predict higher merger rates, but with factor of {approx}2 uncertainties stemming from the uncertainty in those observations. The choice of mass used to define major and minor mergers also matters: stellar-stellar major mergers can be more or less abundant than halo-halo major mergers by an order of magnitude. At low masses, most true major mergers (mass ratio defined in terms of their baryonic or dynamical mass) will appear to be minor mergers in their stellar mass ratio-observations and models using just stellar criteria could underestimate major-merger rates by factors of {approx}3-5. We discuss the uncertainties in relating any merger rate to spheroid formation (in observations or theory): in order to achieve better than factor of {approx}3 accuracy, it is necessary to account for the distribution of merger orbital parameters, gas fractions, and the full efficiency of merger-induced effects as a function of mass ratio.«xa0less