Galactic outflows and the kinematics of damped Lyman alpha absorbers

The kinematics of damped Lyman alpha absorbers (DLAs) are difficult to reproduce in hierarchical galaxy formation models, particularly the preponderance of wide systems. We investigate DLA kinematics at z=3 using high-resolution cosmological hydrodynamical simulations that include a heuristic model for galactic outflows. Without outflows, our simulations fail to yield enough wide DLAs, as in previous studies. With outflows, predicted DLA kinematics are in much better agreement with observations. Comparing two outflow models, we find that a model based on momentum-driven wind scalings provides the best match to the observed DLA kinematic statistics of Prochaska & Wolfe. In this model, DLAs typically arise a few kpc away from galaxies that would be identified in emission. Narrow DLAs can arise from any halo and galaxy mass, but wide ones only arise in halos with mass >10^11 Mo, from either large central or small satellite galaxies. This implies that the success of this outflow model originates from being most efficient at pushing gas out from small satellite galaxies living in larger halos. This increases the cross-section for large halos relative to smaller ones, thereby yielding wider kinematics. Our simulations do not include radiative transfer effects or detailed metal tracking, and outflows are modeled heuristically, but they strongly suggest that galactic outflows are central to understanding DLA kinematics. An interesting consequence is that DLA kinematics may place constraints on the nature and efficiency of gas ejection from high-z galaxies.Comment: submitted to MNRA

Intergalactic Dust Extinction in Hydrodynamic Cosmological Simulations

Recently Menard et al. detected a subtle but systematic change in the mean color of quasars as a function of their projected separation from foreground galaxies, extending to comoving separations of ~10Mpc/h, which they interpret as a signature of reddening by intergalactic dust. We present theoretical models of this remarkable observation, using SPH cosmological simulations of a (50Mpc/h)^3 volume. Our primary model uses a simulation with galactic winds and assumes that dust traces the intergalactic metals. The predicted galaxy-dust correlation function is similar in form to the galaxy-mass correlation function, and reproducing the MSFR data requires a dust-to-metal mass ratio of 0.24, about half the value in the Galactic ISM. Roughly half of the reddening arises in dust that is more than 100Kpc/h from the nearest massive galaxy. We also examine a simulation with no galactic winds, which predicts a much smaller fraction of intergalactic metals (3% vs. 35%) and therefore requires an unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both models, the signal is dominated by sightlines with E(g-i)=0.001-0.1. The no-wind simulation can be reconciled with the data if we also allow reddening to arise in galaxies up to several x 10^10 Msun. The wind model predicts a mean visual extinction of A_V ~0.0133 mag out to z=0.5, with a sightline-to-sightline dispersion similar to the mean, which could be significant for future supernova cosmology studies. Reproducing the MSFR results in these simulations requires that a large fraction of ISM dust survive its expulsion from galaxies and its residence in the intergalactic medium. Future observational studies that provide higher precision and measure the dependence on galaxy type and environment will allow detailed tests for models of enriched galactic outflows and the survival of IG dust.Comment: Matches version accepted by MNRA

How do dwarf galaxies acquire their mass & when do they form their stars?

We apply a simple, one-equation, galaxy formation model on top of the halos and subhalos of a high-resolution dark matter cosmological simulation to study how dwarf galaxies acquire their mass and, for better mass resolution, on over 10^5 halo merger trees, to predict when they form their stars. With the first approach, we show that the large majority of galaxies within group- and cluster-mass halos have acquired the bulk of their stellar mass through gas accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are not built up by galaxy mergers. With the second approach, we constrain the star formation histories of dwarfs by requiring that star formation must occur within halos of a minimum circular velocity set by the evolution of the temperature of the IGM, starting before the epoch of reionization. We qualitatively reproduce the downsizing trend of greater ages at greater masses and predict an upsizing trend of greater ages as one proceeds to masses lower than m_crit. We find that the fraction of galaxies with very young stellar populations (more than half the mass formed within the last 1.5 Gyr) is a function of present-day mass in stars and cold gas, which peaks at 0.5% at m_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. We predict that the baryonic mass function of galaxies should not show a maximum at masses above 10^5.5, M_Sun, and we speculate on the nature of the lowest mass galaxies.Comment: 6 pages, to appear in "A Universe of Dwarf Galaxies: Observations, Theories, Simulations", ed. M. Koleva, P. Prugniel & I. Vauglin, EAS Series (Paris: EDP

The origin of ultra diffuse galaxies: stellar feedback and quenching

We test if the cosmological zoom-in simulations of isolated galaxies from the FIRE project reproduce the properties of ultra diffuse galaxies. We show that stellar feedback-generated outflows that dynamically heat galactic stars, together with a passively aging stellar population after imposed quenching (from e.g. infall into a galaxy cluster), naturally reproduce the observed population of red UDGs, without the need for high spin halos or dynamical influence from their host cluster. We reproduce the range of surface brightness, radius and absolute magnitude of the observed z=0 red UDGs by quenching simulated galaxies at a range of different times. They represent a mostly uniform population of dark matter-dominated galaxies with M_star ~1e8 Msun, low metallicity and a broad range of ages. The most massive simulated UDGs require earliest quenching and are therefore the oldest. Our simulations provide a good match to the central enclosed masses and the velocity dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the simulated UDGs remain largely fixed across a broad range of quenching times because the central regions of their dark matter halos complete their growth early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The most massive red UDG in our sample requires quenching at z~3 when its halo reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0 its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If our simulated dwarfs are not quenched, they evolve into bluer low-surface brightness galaxies with mass-to-light ratios similar to observed field dwarfs. While our simulation sample covers a limited range of formation histories and halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio

Feedback and Recycled Wind Accretion: Assembling the z=0 Galaxy Mass Function

We analyse cosmological hydrodynamic simulations that include observationally-constrained 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 emphasised in recent theoretical studies. Because of the interaction between outflows and gas in and around halos, the recycling timescale of wind material (t_rec) is shorter in higher-mass systems, which reside in denser gaseous environments. In these simulations, this differential recycling plays a central role in shaping the present-day galaxy stellar mass function (GSMF). If we remove all particles that were ever ejected in a wind, then the predicted GSMFs are much steeper than observed; galaxy masses are suppressed both by the direct removal of gas 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 < 5x10^10 Msolar. At higher masses, wind recycling leads to excessive galaxy masses and excessive star formation rates relative to observations. In these massive systems, some quenching mechanism must suppress 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.Comment: 16 pages, 7 figures, accepted by MNRA

SIDM on FIRE: Hydrodynamical Self-Interacting Dark Matter simulations of low-mass dwarf galaxies

We compare a suite of four simulated dwarf galaxies formed in 10$^{10} M_{\odot}$ haloes of collisionless Cold Dark Matter (CDM) with galaxies simulated in the same haloes with an identical galaxy formation model but a non-zero cross-section for dark matter self-interactions. These cosmological zoom-in simulations are part of the Feedback In Realistic Environments (FIRE) project and utilize the FIRE-2 model for hydrodynamics and galaxy formation physics. We find the stellar masses of the galaxies formed in Self-Interacting Dark Matter (SIDM) with $\sigma/m= 1\, cm^2/g$ are very similar to those in CDM (spanning $M_{\star} \approx 10^{5.7 - 7.0} M_{\odot}$) and all runs lie on a similar stellar mass -- size relation. The logarithmic dark matter density slope ($\alpha=d\log \rho / d\log r$) in the central $250-500$ pc remains steeper than $\alpha= -0.8$ for the CDM-Hydro simulations with stellar mass $M_{\star} \sim 10^{6.6} M_{\odot}$ and core-like in the most massive galaxy. In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with $\alpha >-0.4$. Moreover, the central density profiles predicted in SIDM runs without baryons are similar to the SIDM runs that include FIRE-2 baryonic physics. Thus, SIDM appears to be much more robust to the inclusion of (potentially uncertain) baryonic physics than CDM on this mass scale, suggesting SIDM will be easier to falsify than CDM using low-mass galaxies. Our FIRE simulations predict that galaxies less massive than $M_{\star} < 3 \times 10^6 M_{\odot}$ provide potentially ideal targets for discriminating models, with SIDM producing substantial cores in such tiny galaxies and CDM producing cusps.Comment: 10 Pages, 7 figures, submitted to MNRA

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

(Abridged) We develop and test a model for the cosmological role of mergers in the formation and quenching of red, early-type galaxies. Making the 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 at ~L_star, as well as the observed mass functions/density of red galaxies as a function of redshift, the formation times of spheroids as a function of mass, and the fraction of quenched galaxies as a function of galaxy and halo mass, environment, and redshift. Comparing to a variety of semi-analytic models in which quenching is primarily driven by halo mass considerations or secular/disk instabilities, we demonstrate that our model and different broad classes of models make unique and robust qualitative predictions for a number of observables, including the red fraction as a function of galaxy and halo mass, the density of passive galaxies and evolution of the color-morphology-density relations at high z, and the fraction of disky/boxy spheroids as a function of mass. In each case, the observations favor a model in which galaxies quench after a major merger builds a massive spheroid, and disfavor quenching via secular or pure halo processes. We discuss a variety of physical possibilities for this quenching, and propose a mixed scenario in which traditional quenching in hot, massive halos is supplemented by the feedback associated with star formation and quasar activity in a major merger, 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.Comment: 29 pages, 21 figures, submitted to ApJ. Replacement fixes comparison of models in Figures 6 &

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