217 research outputs found
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 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 are very similar to those in CDM
(spanning ) and all runs lie on a
similar stellar mass -- size relation. The logarithmic dark matter density
slope () in the central pc remains
steeper than for the CDM-Hydro simulations with stellar mass
and core-like in the most massive galaxy.
In contrast, every SIDM hydrodynamic simulation yields a flatter profile, with
. 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 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
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