56 research outputs found
How Do Galaxies Get Their Gas?
We examine the temperature history of gas accreted by forming galaxies in SPH
simulations. About half the gas shock heats to roughly the virial temperature
of the galaxy potential well before cooling, condensing, and forming stars, but
the other half radiates its acquired gravitational energy at much lower
temperatures, typically T<10^5 K, and the histogram of maximum gas temperatures
is clearly bimodal. The "cold mode" of gas accretion dominates for low mass
galaxies (M_baryon < 10^{10.3}Msun or M_halo < 10^{11.4}Msun), while the
conventional "hot mode" dominates the growth of high mass systems. Cold
accretion is often directed along filaments, allowing galaxies to efficiently
draw gas from large distances, while hot accretion is quasi-spherical. The
galaxy and halo mass dependence leads to redshift and environment dependence of
cold and hot accretion rates, with cold mode dominating at high redshift and in
low density regions today, and hot mode dominating in group and cluster
environments at low redshift. Star formation rates closely track accretion
rates, and we discuss the physics behind the observed environment and redshift
dependence of galactic scale star formation. If we allowed hot accretion to be
suppressed by conduction or AGN feedback, then the simulation predictions would
change in interesting ways, perhaps resolving conflicts with the colors of
ellipticals and the cutoff of the galaxy luminosity function. The transition
between cold and hot accretion at M_h ~ 10^{11.4}Msun is similar to that found
by Birnboim & Dekel (2003) using 1-d simulations and analytic arguments. The
corresponding baryonic mass is tantalizingly close to the scale at which
Kauffmann et al. (2003) find a marked shift in galaxy properties. We speculate
on connections between these theoretical and observational transitions.Comment: 1 figure added, Appendix discussing SAMs added, some text changes.
Matches the version accepted by MNRAS. 31 pages (MNRAS style), 21 figures,For
high resolution version of the paper (highly recommended) follow
http://www.astro.umass.edu/~keres/paper/ms2.ps.g
The formation of massive, quiescent galaxies at cosmic noon
The cosmic noon (z~1.5-3) marked a period of vigorous star formation for most
galaxies. However, about a third of the more massive galaxies at those times
were quiescent in the sense that their observed stellar populations are
inconsistent with rapid star formation. The reduced star formation activity is
often attributed to gaseous outflows driven by feedback from supermassive black
holes, but the impact of black hole feedback on galaxies in the young Universe
is not yet definitively established. We analyze the origin of quiescent
galaxies with the help of ultra-high resolution, cosmological simulations that
include feedback from stars but do not model the uncertain consequences of
black hole feedback. We show that dark matter halos with specific accretion
rates below ~0.25-0.4 per Gyr preferentially host galaxies with reduced star
formation rates and red broad-band colors. The fraction of such halos in large
dark matter only simulations matches the observed fraction of massive quiescent
galaxies (~10^10-10^11 Msun). This strongly suggests that halo accretion rate
is the key parameter determining which massive galaxies at z~1.5-3 become
quiescent. Empirical models that connect galaxy and halo evolution, such as
halo occupation distribution or abundance matching models, assume a tight link
between galaxy properties and the masses of their parent halos. These models
will benefit from adding the specific accretion rate of halos as a second model
parameter.Comment: 5 pages, 5 figures, to appear in MNRAS Letter
Moving mesh cosmology: tracing cosmological gas accretion
We investigate the nature of gas accretion onto haloes and galaxies at z=2
using cosmological hydrodynamic simulations run with the moving mesh code
AREPO. Implementing a Monte Carlo tracer particle scheme to determine the
origin and thermodynamic history of accreting gas, we make quantitative
comparisons to an otherwise identical simulation run with the smoothed particle
hydrodynamics (SPH) code GADGET-3. Contrasting these two numerical approaches,
we find significant physical differences in the thermodynamic history of
accreted gas in haloes above 10^10.5 solar masses. In agreement with previous
work, GADGET simulations show a cold fraction near unity for galaxies forming
in massive haloes, implying that only a small percentage of accreted gas heats
to an appreciable fraction of the virial temperature during accretion. The same
galaxies in AREPO show a much lower cold fraction, <20% in haloes above 10^11
solar masses. This results from a hot gas accretion rate which, at this same
halo mass, is an order of magnitude larger than with GADGET, while the cold
accretion rate is also lower. These discrepancies increase for more massive
systems, and we explain both as due to numerical inaccuracies in the standard
formulation of SPH. We also observe that the relatively sharp transition from
cold to hot mode dominated accretion, at a halo mass of ~10^11, is a
consequence of comparing past gas temperatures to a constant threshold value
independent of virial temperature. Examining the spatial distribution of
accreting gas, we find that gas filaments in GADGET tend to remain collimated
and flow coherently to small radii, or artificially fragment and form a large
number of purely numerical "blobs". Similar gas streams in AREPO show increased
heating and disruption at 0.25-0.5 virial radii and contribute to the hot gas
accretion rate in a manner distinct from classical cooling flows.Comment: 21 pages, 12 figures. MNRAS accepted (in press). High-resolution
images can be found at
http://www.cfa.harvard.edu/itc/research/movingmeshcosmology
Galactic r-process enrichment by neutron star mergers in cosmological simulations of a Milky Way-mass galaxy
We quantify the stellar abundances of neutron-rich r-process nuclei in
cosmological zoom-in simulations of a Milky Way-mass galaxy from the Feedback
In Realistic Environments project. The galaxy is enriched with r-process
elements by binary neutron star (NS) mergers and with iron and other metals by
supernovae. These calculations include key hydrodynamic mixing processes not
present in standard semi-analytic chemical evolution models, such as galactic
winds and hydrodynamic flows associated with structure formation. We explore a
range of models for the rate and delay time of NS mergers, intended to roughly
bracket the wide range of models consistent with current observational
constraints. We show that NS mergers can produce [r-process/Fe] abundance
ratios and scatter that appear reasonably consistent with observational
constraints. At low metallicity, [Fe/H]<-2, we predict there is a wide range of
stellar r-process abundance ratios, with both supersolar and subsolar
abundances. Low-metallicity stars or stars that are outliers in their r-process
abundance ratios are, on average, formed at high redshift and located at large
galactocentric radius. Because NS mergers are rare, our results are not fully
converged with respect to resolution, particularly at low metallicity. However,
the uncertain rate and delay time distribution of NS mergers introduces an
uncertainty in the r-process abundances comparable to that due to finite
numerical resolution. Overall, our results are consistent with NS mergers being
the source of most of the r-process nuclei in the Universe.Comment: Accepted for publication in MNRAS, 10 pages and 4 figures. Revised
version: minor change
Giant clumps in the FIRE simulations: a case study of a massive high-redshift galaxy
The morphology of massive star-forming galaxies at high redshift is often
dominated by giant clumps of mass ~10^8-10^9 Msun and size ~100-1000 pc.
Previous studies have proposed that giant clumps might have an important role
in the evolution of their host galaxy, particularly in building the central
bulge. However, this depends on whether clumps live long enough to migrate from
their original location in the disc or whether they get disrupted by their own
stellar feedback before reaching the centre of the galaxy. We use cosmological
hydrodynamical simulations from the FIRE (Feedback in Realistic Environments)
project that implement explicit treatments of stellar feedback and ISM physics
to study the properties of these clumps. We follow the evolution of giant
clumps in a massive (stellar mass ~10^10.8 Msun at z=1), discy, gas-rich galaxy
from redshift z>2 to z=1. Even though the clumpy phase of this galaxy lasts
over a gigayear, individual gas clumps are short-lived, with mean lifetime of
massive clumps of ~20 Myr. During that time, they turn between 0.1% and 20% of
their gas into stars before being disrupted, similar to local GMCs. Clumps with
M>10^7 Msun account for ~20% of the total star formation in the galaxy during
the clumpy phase, producing ~10^10 Msun of stars. We do not find evidence for
net inward migration of clumps within the galaxy. The number of giant clumps
and their mass decrease at lower redshifts, following the decrease in the
overall gas fraction and star-formation rate.Comment: 20 pages, 19 figures; revised version, accepted for publication in
MNRA
Reconciling dwarf galaxies with LCDM cosmology: Simulating a realistic population of satellites around a Milky Way-mass galaxy
Low-mass "dwarf" galaxies represent the most significant challenges to the
cold dark matter (CDM) model of cosmological structure formation. Because these
faint galaxies are (best) observed within the Local Group (LG) of the Milky Way
(MW) and Andromeda (M31), understanding their formation in such an environment
is critical. We present first results from the Latte Project: the Milky Way on
FIRE (Feedback in Realistic Environments). This simulation models the formation
of a MW-mass galaxy to z = 0 within LCDM cosmology, including dark matter, gas,
and stars at unprecedented resolution: baryon particle mass of 7070 Msun with
gas kernel/softening that adapts down to 1 pc (with a median of 25 - 60 pc at z
= 0). Latte was simulated using the GIZMO code with a mesh-free method for
accurate hydrodynamics and the FIRE-2 model for star formation and explicit
feedback within a multi-phase interstellar medium. For the first time, Latte
self-consistently resolves the spatial scales corresponding to half-light radii
of dwarf galaxies that form around a MW-mass host down to Mstar > 10^5 Msun.
Latte's population of dwarf galaxies agrees with the LG across a broad range of
properties: (1) distributions of stellar masses and stellar velocity
dispersions (dynamical masses), including their joint relation; (2) the
mass-metallicity relation; and (3) a diverse range of star-formation histories,
including their mass dependence. Thus, Latte produces a realistic population of
dwarf galaxies at Mstar > 10^5 Msun that does not suffer from the "missing
satellites" or "too big to fail" problems of small-scale structure formation.
We conclude that baryonic physics can reconcile observed dwarf galaxies with
standard LCDM cosmology.Comment: 7 pages, 5 figures. Accepted for publication in ApJ Letters. Several
updates, including: (1) fixed a bug in halo finder, now identifies 13
satellite galaxies and more subhalos in the baryonic simulation; (2) fixed a
minor bug in the feedback coupling and reran the simulation, resulting in a
somewhat lower-mass host galaxy; (3) Fig 2 now shows stellar velocity
dispersion profiles of satellite
The Difficulty of Getting High Escape Fractions of Ionizing Photons from High-redshift Galaxies: a View from the FIRE Cosmological Simulations
We present a series of high-resolution (20-2000 Msun, 0.1-4 pc) cosmological
zoom-in simulations at z~6 from the Feedback In Realistic Environment (FIRE)
project. These simulations cover halo masses 10^9-10^11 Msun and rest-frame
ultraviolet magnitude Muv = -9 to -19. These simulations include explicit
models of the multi-phase ISM, star formation, and stellar feedback, which
produce reasonable galaxy properties at z = 0-6. We post-process the snapshots
with a radiative transfer code to evaluate the escape fraction (fesc) of
hydrogen ionizing photons. We find that the instantaneous fesc has large time
variability (0.01%-20%), while the time-averaged fesc over long time-scales
generally remains ~5%, considerably lower than the estimate in many
reionization models. We find no strong dependence of fesc on galaxy mass or
redshift. In our simulations, the intrinsic ionizing photon budgets are
dominated by stellar populations younger than 3 Myr, which tend to be buried in
dense birth clouds. The escaping photons mostly come from populations between
3-10 Myr, whose birth clouds have been largely cleared by stellar feedback.
However, these populations only contribute a small fraction of intrinsic
ionizing photon budgets according to standard stellar population models. We
show that fesc can be boosted to high values, if stellar populations older than
3 Myr produce more ionizing photons than standard stellar population models (as
motivated by, e.g., models including binaries). By contrast, runaway stars with
velocities suggested by observations can enhance fesc by only a small fraction.
We show that "sub-grid" star formation models, which do not explicitly resolve
star formation in dense clouds with n >> 1 cm^-3, will dramatically
over-predict fesc.Comment: 17 pages, 16 figures, MNRAS in pres
The Origin and Evolution of the Galaxy Mass-Metallicity Relation
We use high-resolution cosmological zoom-in simulations from the Feedback in
Realistic Environment (FIRE) project to study the galaxy mass-metallicity
relations (MZR) from z=0-6. These simulations include explicit models of the
multi-phase ISM, star formation, and stellar feedback. The simulations cover
halo masses Mhalo=10^9-10^13 Msun and stellar mass Mstar=10^4-10^11 Msun at z=0
and have been shown to produce many observed galaxy properties from z=0-6. For
the first time, our simulations agree reasonably well with the observed
mass-metallicity relations at z=0-3 for a broad range of galaxy masses. We
predict the evolution of the MZR from z=0-6 as
log(Zgas/Zsun)=12+log(O/H)-9.0=0.35[log(Mstar/Msun)-10]+0.93 exp(-0.43 z)-1.05
and log(Zstar/Zsun)=[Fe/H]-0.2=0.40[log(Mstar/Msun)-10]+0.67 exp(-0.50 z)-1.04,
for gas-phase and stellar metallicity, respectively. Our simulations suggest
that the evolution of MZR is associated with the evolution of stellar/gas mass
fractions at different redshifts, indicating the existence of a universal
metallicity relation between stellar mass, gas mass, and metallicities. In our
simulations, galaxies above Mstar=10^6 Msun are able to retain a large fraction
of their metals inside the halo, because metal-rich winds fail to escape
completely and are recycled into the galaxy. This resolves a long-standing
discrepancy between "sub-grid" wind models (and semi-analytic models) and
observations, where common sub-grid models cannot simultaneously reproduce the
MZR and the stellar mass functions.Comment: 17 pages, 14 figures, re-submitted to MNRAS after revisions on
referee comment
The creation and persistence of a misaligned gas disc in a simulated early-type galaxy
Massive early-type galaxies commonly have gas discs which are kinematically
misaligned with the stellar component. These discs feel a torque from the stars
and the angular momentum vectors are expected to align quickly. We present
results on the evolution of a misaligned gas disc in a cosmological simulation
of a massive early-type galaxy from the Feedback In Realistic Environments
project. This galaxy experiences a merger which, together with a strong
galactic wind, removes most of the original gas disc. The galaxy subsequently
reforms a gas disc through accretion of cold gas, but it is initially 120
degrees misaligned with the stellar rotation axis. This misalignment persists
for about 2 Gyr before the gas-star misalignment angle drops below 20 degrees.
The time it takes for the gaseous and stellar components to align is much
longer than previously thought, because the gas disc is accreting a significant
amount of mass for about 1.5 Gyr after the merger, during which the angular
momentum change induced by accreted gas dominates over that induced by stellar
torques. Once the gas accretion rate has decreased sufficiently, the gas disc
decouples from the surrounding halo gas and realigns with the stellar component
in about 6 dynamical times. During the late evolution of the misaligned gas
disc, the centre aligns faster than the outskirts, resulting in a warped disc.
We discuss the observational consequences of the long survival of our
misaligned gas disc and how our results can be used to calibrate merger rate
estimates from observed gas misalignments.Comment: 10 pages, 7 figures. Accepted for publication in MNRAS. Revised
version: minor changes. A movie of the evolution of the gas disc can be
viewed at http://astro.berkeley.edu/~freeke/misalign.htm
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