Dušan Kereš

Bio: Dušan Kereš is an academic researcher from University of California, San Diego. The author has contributed to research in topics: Galaxy & Galaxy formation and evolution. The author has an hindex of 40, co-authored 79 publications receiving 13524 citations. Previous affiliations of Dušan Kereš include University of California, Berkeley & CFA Institute.

How do galaxies get their gas

Abstract: 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

Abstract: 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.

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

Abstract: (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.

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

Abstract: 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

Galaxies in a simulated $\Lambda$CDM Universe I: cold mode and hot cores

Abstract: We study the formation of galaxies in a (50 Mpc/h)^3 cosmological simulation (2x288^3 particles), evolved using the entropy conserving 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 (Keres et al. 2005). Atmospheres of hot, virialized gas develop in halos above ~2.5e11 Msun, a transition mass that is nearly constant from z=3 to z=0. Cold accretion persists in halos above the transition mass, especially at z>=2. It dominates the growth of galaxies in low mass halos at all times, and it is the main driver of the cosmic star formation history. Satellite galaxies have accretion rates similar to central galaxies of the same baryonic mass at high redshifts, but they have less accretion than comparable central galaxies at low redshift. Relative to our earlier results, the Gadget-2 simulations predict much lower rates of "hot mode" accretion from the virialized gas component of massive halos. At z<=1, typical hot accretion rates in halos above 5e12 Msun are below 1 Msun/yr, even though our simulation does not include AGN heating or other forms of "preventive" feedback. The inner density profiles of hot gas in these halos are shallow, with long associated cooling times. The cooling recipes typically used in semi-analytic models can overestimate the accretion rates in these halos by orders of magnitude, so such models may overemphasize the role of preventive feedback in producing observed galaxy masses and colors. A fraction of the massive halos develop cuspy profiles and significant cooling rates between z=1 and z=0, a redshift trend similar to the observed trend in the frequency of cooling flow clusters.

Simulations of the formation, evolution and clustering of galaxies and quasars

Abstract: The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a simulation of the growth of dark matter structure using 2,1603 particles, following them from redshift z = 127 to the present in a cube-shaped region 2.230 billion lightyears on a side. In postprocessing, we also follow the formation and evolution of the galaxies and quasars. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies.

The Observation of Gravitational Waves from a Binary Black Hole Merger

Abstract: On September 14, 2015 at 09:50:45 UTC the two detectors of the Laser Interferometer Gravitational-Wave Observatory simultaneously observed a transient gravitational-wave signal. The signal sweeps upwards in frequency from 35 to 250 Hz with a peak gravitational-wave strain of 1.0×10(-21). It matches the waveform predicted by general relativity for the inspiral and merger of a pair of black holes and the ringdown of the resulting single black hole. The signal was observed with a matched-filter signal-to-noise ratio of 24 and a false alarm rate estimated to be less than 1 event per 203,000 years, equivalent to a significance greater than 5.1σ. The source lies at a luminosity distance of 410(-180)(+160) Mpc corresponding to a redshift z=0.09(-0.04)(+0.03). In the source frame, the initial black hole masses are 36(-4)(+5)M⊙ and 29(-4)(+4)M⊙, and the final black hole mass is 62(-4)(+4)M⊙, with 3.0(-0.5)(+0.5)M⊙c(2) radiated in gravitational waves. All uncertainties define 90% credible intervals. These observations demonstrate the existence of binary stellar-mass black hole systems. This is the first direct detection of gravitational waves and the first observation of a binary black hole merger.

Cosmic Star-Formation History

Abstract: Over the past two decades, an avalanche of data from multiwavelength imaging and spectroscopic surveys has revolutionized our view of galaxy formation and evolution. Here we review the range of complementary techniques and theoretical tools that allow astronomers to map the cosmic history of star formation, heavy element production, and reionization of the Universe from the cosmic "dark ages" to the present epoch. A consistent picture is emerging, whereby the star-formation rate density peaked approximately 3.5 Gyr after the Big Bang, at z~1.9, and declined exponentially at later times, with an e-folding timescale of 3.9 Gyr. Half of the stellar mass observed today was formed before a redshift z = 1.3. About 25% formed before the peak of the cosmic star-formation rate density, and another 25% formed after z = 0.7. Less than ~1% of today's stars formed during the epoch of reionization. Under the assumption of a universal initial mass function, the global stellar mass density inferred at any epoch matches reasonably well the time integral of all the preceding star-formation activity. The comoving rates of star formation and central black hole accretion follow a similar rise and fall, offering evidence for co-evolution of black holes and their host galaxies. The rise of the mean metallicity of the Universe to about 0.001 solar by z = 6, one Gyr after the Big Bang, appears to have been accompanied by the production of fewer than ten hydrogen Lyman-continuum photons per baryon, a rather tight budget for cosmological reionization.

The EAGLE project: Simulating the evolution and assembly of galaxies and their environments

Abstract: We introduce the Virgo Consortium's EAGLE project, a suite of hydrodynamical simulations that follow the formation of galaxies and black holes in representative volumes. We discuss the limitations of such simulations in light of their finite resolution and poorly constrained subgrid physics, and how these affect their predictive power. One major improvement is our treatment of feedback from massive stars and AGN in which thermal energy is injected into the gas without the need to turn off cooling or hydrodynamical forces, allowing winds to develop without predetermined speed or mass loading factors. Because the feedback efficiencies cannot be predicted from first principles, we calibrate them to the z~0 galaxy stellar mass function and the amplitude of the galaxy-central black hole mass relation, also taking galaxy sizes into account. The observed galaxy mass function is reproduced to ≲0.2 dex over the full mass range, 108

Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies

Abstract: Supermassive black holes (BHs) have been found in 85 galaxies by dynamical modeling of spatially resolved kinematics. The Hubble Space Telescope revolutionized BH research by advancing the subject from its proof-of-concept phase into quantitative studies of BH demographics. Most influential was the discovery of a tight correlation between BH mass and the velocity dispersion σ of the bulge component of the host galaxy. Together with similar correlations with bulge luminosity and mass, this led to the widespread belief that BHs and bulges coevolve by regulating each other's growth. Conclusions based on one set of correlations from in brightest cluster ellipticals to in the smallest galaxies dominated BH work for more than a decade. New results are now replacing this simple story with a richer and more plausible picture in which BHs correlate differently with different galaxy components. A reasonable aim is to use this progress to refine our understanding of BH-galaxy coevolution. BHs with masses of 105−106M...