Romeel Davé

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.

Baryons in the Warm-Hot Intergalactic Medium

Approximately 30%-40% of all baryons in the present-day universe reside in a warm-hot intergalactic medium (WHIM), with temperatures in the range 105 < T < 107 K. This is a generic prediction from six hydrodynamic simulations of currently favored structure formation models having a wide variety of numerical methods, input physics, volumes, and spatial resolutions. Most of these warm-hot baryons reside in diffuse large-scale structures with a median overdensity around 10-30, not in virialized objects such as galaxy groups or galactic halos. The evolution of the WHIM is primarily driven by shock heating from gravitational perturbations breaking on mildly nonlinear, nonequilibrium structures such as filaments. Supernova feedback energy and radiative cooling play lesser roles in its evolution. WHIM gas may be consistent with observations of the 0.25 keV X-ray background without being significantly heated by nongravitational processes because the emitting gas is very diffuse. Our results confirm and extend previous work by Cen & Ostriker and Dave et al.

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

Cosmological simulations of intergalactic medium enrichment from galactic outflows

We investigate models of self-consistent chemical enrichment of the intergalactic medium (IGM) from z = 6.0 → 1.5, based on hydrodynamic simulations of structure formation that explicitly incorporate outflows from star-forming galaxies. Our main result is that outflow parametrizations derived from observations of local starburst galaxies, in particular momentum-driven wind scenarios, provide the best agreement with observations of C iv absorption at z ∼ 2-5. Such models sufficiently enrich the high-z IGM to produce a global mass density of Civ absorbers that is relatively invariant from z = 5.5 → 1.5, in agreement with observations. This occurs despite continual IGM enrichment causing an increase in volume-averaged metallicity by ∼ x 5-10 over this redshift range, because energy input accompanying the enriching outflows causes a drop in the global ionization fraction of C iv. Comparisons to observed C IV column density and linewidth distributions and C IV-based pixel optical depth ratios provide significant constraints on wind models. Our best-fitting outflow models show mean IGM temperatures only slightly above our no-outflow case, metal filling factors of just a few per cent with volume-weighted metallicities around 10 -3 at z ∼ 3, significant amounts of collisionally ionized C iv absorption and a metallicity-density relationship that rises rapidly at low overdensities and flattens at higher ones. In general, we find that outflow speeds must be high enough to enrich the low-density IGM at early times but low enough not to overheat it, and concurrently must significantly suppress early star formation while still producing enough early metals. It is therefore non-trivial that locally calibrated momentum-driven wind scenarios naturally yield the desired strength and evolution of outflows, and suggest that such models represent a significant step towards understanding the impact of galactic outflows on galaxies and the IGM across cosmic time.

The low-redshift Lyα forest in cold dark matter cosmologies

We study the physical origin of the low-redshift Lyα forest in hydrodynamic simulations of four cosmological models, all variants of the cold dark matter scenario. Our most important conclusions are insensitive to the cosmological model, but they depend on our assumption that the UV background declines at low redshift in concert with the declining population of quasar sources. We find that the expansion of the universe drives rapid evolution of dN/dz (the number of absorbers per unit redshift above a specified equivalent width threshold) at z1.7, but that at lower redshift the fading of the UV background counters the influence of expansion, leading to slow evolution of dN/dz. The draining of gas from low-density regions into collapsed structures has a mild but not negligible effect on the evolution of dN/dz, especially for high equivalent-width thresholds. At every redshift, weaker lines come primarily from moderate fluctuations of the diffuse, unshocked intergalactic medium (IGM) and stronger lines originate in shocked or radiatively cooled gas of higher overdensity. However, the neutral hydrogen column density associated with structures of fixed overdensity drops as the universe expands, so an absorber at z=0 is dynamically analogous to an absorber that has column density 10-50 times higher at z=2-3. In particular, the mildly overdense IGM fluctuations that dominate the Lyα forest opacity at z>2 produce optically thin lines at z<1, while the marginally saturated (NH I~1014.5 cm−2) lines at z<1 typically arise in gas that is overdense by a factor of 20-100. We find no clear distinction between lines arising in galaxy halos and lines arising in larger scale structures; however, galaxies tend to lie near the dense regions of the IGM that are responsible for strong Lyα lines. The simulations provide a unified physical picture that accounts for the most distinctive observed properties of the low-redshift Lyα forest: (1) a sharp transition in the evolution of dN/dz at z~1.7, (2) stronger evolution for absorbers of higher equivalent width, (3) a correlation of increasing Lyα equivalent width with decreasing galaxy impact parameter that extends to rp~500 h−1 kpc, and (4) a tendency for stronger lines to arise in close proximity to galaxies while weaker lines trace more diffuse large-scale structure.

Mass, Metal, and Energy Feedback in Cosmological Simulations

Using GADGET-2 cosmological hydrodynamic simulations including an observationally constrained model for galactic outflows, we investigate how feedback from star formation distributes mass, metals, and energy on cosmic scales from z = 6 → 0. We include instantaneous enrichment from Type II supernovae (SNe), as well as delayed enrichment from Type Ia SNe and stellar [asymptotic giant branch (AGB)] mass loss, and we individually track carbon, oxygen, silicon and iron using the latest yields. Following on the success of the momentum-driven wind scalings, we improve our implementation by using an on-the-fly galaxy finder to derive wind properties based on host galaxy masses. By tracking wind particles in a suite of simulations, we find: (1) wind material re-accretes on to a galaxy (usually the same one it left) on a recycling time-scale that varies inversely with galaxy mass (e.g. <1 Gyr for L ∗ galaxies at

An Analytic Model for the Evolution of the Stellar, Gas, and Metal Content of Galaxies

We present an analytic formalism that describes the evolution of the stellar, gas and metal content of galaxies. It is based on the idea, inspired by hydrodynamic simulations, that galaxies live in a slowly evolving equilibrium between inflow, outflow and star formation. We argue that this formalism broadly captures the behaviour of galaxy properties evolving in simulations. The resulting equilibrium equations for the star formation rate, gas fraction and metallicity depend on three key free parameters that represent ejective feedback, preventive feedback and reaccretion of ejected material. We schematically describe how these parameters are constrained by models and observations. Galaxies perturbed off the equilibrium relations owing to inflow stochasticity tend to be driven back towards equilibrium, such that deviations in star formation rate at a given mass are correlated with gas fraction and anticorrelated with metallicity. After an early gas accumulation epoch, quiescently star-forming galaxies are expected to be in equilibrium over most of cosmic time. The equilibrium model provides a simple intuitive framework for understanding the cosmic evolution of galaxy properties, and centrally features the cycle of baryons between galaxies and surrounding gas as the driver of galaxy growth.

The halo occupation distribution and the physics of galaxy formation

The halo occupation distribution (HOD) describes the bias between galaxies and dark matter by specifying (1) the probability P(N|M) that a halo of virial mass M contains N galaxies of a particular class and (2) the relative spatial and velocity distributions of galaxies and dark matter within halos. We calculate and compare the HODs predicted by a smoothed particle hydrodynamics (SPH) simulation of a ΛCDM cosmological model (cold dark matter with a cosmological constant) and by a semianalytic galaxy formation model applied to the same cosmology. Although the two methods predict different galaxy mass functions, their HOD predictions for samples of the same space density agree remarkably well. In a sample defined by a baryonic mass threshold, the mean occupation function NM exhibits a sharp cutoff at low halo masses, a slowly rising plateau in which N climbs from 1 to 2 over nearly a decade in halo mass, and a more steeply rising high-occupancy regime at high halo mass. In the low-occupancy regime, the factorial moments N(N - 1) and N(N - 1)(N - 2) are well below the values of N2 and N3 expected for Poisson statistics, with important consequences for the small-scale behavior of the two- and three-point correlation functions. The HOD depends strongly on galaxy age, with high-mass halos populated mainly by old galaxies and low-mass halos by young galaxies. The distribution of galaxies within SPH halos supports the assumptions usually made in semianalytic calculations: the most massive galaxy lies close to the halo center and moves near the halos mean velocity, while the remaining, satellite galaxies have the same radial profile and velocity dispersion as the dark matter. The mean occupation at fixed halo mass in the SPH simulation is independent of the halos larger scale environment, supporting both the merger tree approach of the semianalytic method and the claim that the HOD provides a complete statistical characterization of galaxy bias. We discuss the connections between the predicted HODs and the galaxy formation physics incorporated in the SPH and semianalytic approaches. These predictions offer useful guidance to theoretical models of galaxy clustering, and they will be tested empirically by ongoing analyses of galaxy redshift surveys. By applying the HODs to a large-volume N-body simulation, we show that both methods predict slight departures from a power-law galaxy correlation function, similar to features detected in recent observational analyses.

Galaxy evolution in cosmological simulations with outflows – II. Metallicities and gas fractions

We use cosmological hydrodynamic simulations to investigate how inflows, star formation, and outflows govern the the gaseous and metal content of galaxies within a hierarchical structure formation context. In our simulations, galaxy metallicities are established by a balance between inflows and outflows as governed by the mass outflow rate, implying that the mass-metallicity relation reflects how the outflow rate varies with stellar mass. Gas content, meanwhile, is set by a competition between inflow into and gas consumption within the interstellar medium, the latter being governed by the star formation law, while the former is impacted by both wind recycling and preventive feedback. Stochastic variations in the inflow rate move galaxies off the equilibrium mass-metallicity and mass-gas fraction relations in a manner correlated with star formation rate, and the scatter is set by the timescale to re-equilibrate. The evolution of both relations from z = 3 ! 0 is slow, as individual galaxies tend to evolve mostly along the relations. Gas fractions at a given stellar mass slowly decrease with time because the cosmic inflow rate diminishes faster than the consumption rate, while metallicities slowly increase as infalling gas becomes more enriched. Observations from z � 3 ! 0 are better matched by simulations employing momentum-driven wind scalings rather than constant wind speeds, but all models predict too low gas fractions at low masses and too high metallicities at high masses. All our models reproduce observed second-parameter trends of the mass-metallicity relation with star formation rate and environment, indicating that these are a consequence of equilibrium and not feedback. Overall, the analytical framework of our equilibrium scenario broadly captures the relevant physics establishing the galaxy gas and metal content in simulations, which suggests that the cycle of baryonic inflows and outflows centrally governs the cosmic evolution of these properties in typical star-forming galaxies.

Galaxy Evolution in Cosmological Simulations With Outflows I: Stellar Masses and Star Formation Rates

We examine the growth of the stellar content of galaxies from z = 3 → 0 in cosmological hydrodynamic simulations incorporating parametrized galactic outflows. Without outflows, galaxies overproduce stellar masses (M * ) and star formation rates (SFRs) compared to observations. Winds introduce a three-tier form for the galaxy stellar mass and star formation rate functions, where the middle tier depends on the differential (i.e. mass-dependent) recycling of ejected wind material back into galaxies. A tight M * ―SFR relation is a generic outcome of all these simulations and its evolution is well described as being powered by cold accretion, although current observations at z ≳ 2 suggest that the star formation in small early galaxies must be highly suppressed. Roughly, one-third of z = 0 galaxies at masses below M * are satellites and the star formation in satellites is not much burstier than in centrals. All models fail to suppress the star formation and stellar mass growth in massive galaxies at z ≲ 2, indicating the need for an external quenching mechanism such as black hole feedback. All models also fail to produce dwarfs as young and rapidly star forming as observed. An outflow model following scalings expected for momentum-driven winds broadly matches the observed galaxy evolution around M * from z = 0 to 3, which is a significant success since these galaxies dominate cosmic star formation, but the failures at higher and lower masses highlight the challenges still faced by this class of models. We argue that central star-forming galaxies are well described as living in a slowly evolving equilibrium between inflows from gravity and recycled winds, star formation, and strong and ubiquitous outflows that regulate how much inflow forms into stars. Star-forming galaxy evolution is thus primarily governed by the continual cycling of baryons between galaxies and intergalactic gas.