54 research outputs found
Galaxy growth from redshift 5 to 0 at fixed comoving number density
Studying the average properties of galaxies at a fixed comoving number
density over a wide redshift range has become a popular observational method,
because it may trace the evolution of galaxies statistically. We test this
method by comparing the evolution of galaxies at fixed number density and by
following individual galaxies through cosmic time (z=0-5) in cosmological,
hydrodynamical simulations from OWLS. Comparing progenitors, descendants, and
galaxies selected at fixed number density at each redshift, we find differences
of up to a factor of three for galaxy and interstellar medium (ISM) masses. The
difference is somewhat larger for black hole masses. The scatter in ISM mass
increases significantly towards low redshift with all selection techniques. We
use the fixed number density technique to study the assembly of dark matter,
gas, stars, and black holes and the evolution in accretion and star formation
rates. We find three different regimes for massive galaxies, consistent with
observations: at high redshift the gas accretion rate dominates, at
intermediate redshifts the star formation rate is the highest, and at low
redshift galaxies grow mostly through mergers. Quiescent galaxies have much
lower ISM masses (by definition) and much higher black hole masses, but the
stellar and halo masses are fairly similar. Without active galactic nucleus
(AGN) feedback, massive galaxies are dominated by star formation down to z=0
and most of their stellar mass growth occurs in the centre. With AGN feedback,
stellar mass is only added to the outskirts of galaxies by mergers and they
grow inside-out.Comment: Accepted for publication in MNRAS. First submitted on June 19, 201
Characterizing simulated galaxy stellar mass histories
Cosmological galaxy formation simulations can now produce rich and diverse ensembles of galaxy histories. These simulated galaxy histories, taken all together, provide an answer to the question ‘How do galaxies form?’ for the models used to construct them. We characterize such galaxy history ensembles both to understand their properties and to identify points of comparison for histories within a given galaxy formation model or between different galaxy formation models and simulations. We focus primarily on stellar mass histories of galaxies with the same final stellar mass, for six final stellar mass values and for three different simulated galaxy formation models (a semi-analytic model built upon the dark matter Millennium simulation and two models from the hydrodynamical OverWhelmingly Large Simulations project). Using principal component analysis (PCA) to classify scatter around the average stellar mass history, we find that one fluctuation dominates for all sets of histories we consider, although its shape and contribution can vary between samples. We correlate the PCA characterization with several z = 0 galaxy properties (to connect with survey observables) and also compare it to some other galaxy history properties. We then explore separating galaxy stellar mass histories into classes, using the largest PCA contribution, k-means clustering, and simple Gaussian mixture models. For three component models, these different methods often gave similar results. These history classification methods provide a succinct and often quick way to characterize changes in the full ensemble of histories of a simulated population as physical assumptions are varied, to compare histories of different simulated populations to each other, and to assess the relation of simulated histories to fixed time observations
Stellar initial mass function variation in massive early-type galaxies: the potential role of the deuterium abundance
The observed stellar initial mass function (IMF) appears to vary, becoming
bottom-heavy in the centres of the most massive, metal-rich early-type
galaxies. It is still unclear what physical processes might cause this IMF
variation. In this paper, we demonstrate that the abundance of deuterium in the
birth clouds of forming stars may be important in setting the IMF. We use
models of disc accretion onto low-mass protostars to show that those forming
from deuterium-poor gas are expected to have zero-age main sequence masses
significantly lower than those forming from primordial (high deuterium
fraction) material. This deuterium abundance effect depends on stellar mass in
our simple models, such that the resulting IMF would become bottom-heavy - as
seen in observations. Stellar mass loss is entirely deuterium-free and is
important in fuelling star formation across cosmic time. Using the EAGLE
simulation we show that stellar mass loss-induced deuterium variations are
strongest in the same regions where IMF variations are observed: at the centres
of the most massive, metal-rich, passive galaxies. While our analysis cannot
prove that the deuterium abundance is the root cause of the observed IMF
variation, it sets the stage for future theoretical and observational attempts
to study this possibility.Comment: 10 pages, 5 figures, accepted to MNRA
Cosmological simulations of the circumgalactic medium with 1 kpc resolution: enhanced HI column densities
The circumgalactic medium (CGM), i.e. the gaseous haloes around galaxies, is
both the reservoir of gas that fuels galaxy growth and the repository of gas
expelled by galactic winds. Most cosmological, hydrodynamical simulations focus
their computational effort on the galaxies themselves and treat the CGM more
coarsely, which means small-scale structure cannot be resolved. We get around
this issue by running zoom-in simulations of a Milky Way-mass galaxy with
standard mass refinement and additional uniform spatial refinement within the
virial radius. This results in a detailed view of its gaseous halo at
unprecedented (1 kpc) uniform resolution with only a moderate increase in
computational time. The improved spatial resolution does not impact the central
galaxy or the average density of the CGM. However, it drastically changes the
radial profile of the neutral hydrogen column density, which is enhanced at
galactocentric radii larger than 40 kpc. The covering fraction of Lyman-Limit
Systems within 150 kpc is almost doubled. We therefore conclude that some of
the observational properties of the CGM are strongly resolution dependent.
Increasing the resolution in the CGM, without increasing the resolution of the
galaxies, is a promising and computationally efficient method to push the
boundaries of state-of-the-art simulations.Comment: Accepted for publication in MNRAS Letters. Revised version: minor
change
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
Hydrodynamical simulations of the galaxy population: Enduring successes and outstanding challenges
We review the progress in modeling the galaxy population in hydrodynamical simulations of the ΛCDM cosmogony. State-of-the-art simulations now broadly reproduce the observed spatial clustering of galaxies; the distributions of key characteristics, such as mass, size, and SFR; and scaling relations connecting diverse properties to mass. Such improvements engender confidence in the insight drawn from simulations. Many important outcomes, however, particularly the properties of circumgalactic gas, are sensitive to the details of the subgrid models used to approximate the macroscopic effects of unresolved physics, such as feedback processes. We compare the outcomes of leading simulation suites with observations, and with each other, to identify the enduring successes they have cultivated and the outstanding challenges to be tackled with the next generation of models. Our key conclusions include the following:
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Realistic galaxies can be reproduced by calibrating the ill-constrained parameters of subgrid feedback models. Feedback is dominated by stars and black holes in low-mass and high-mass galaxies, respectively.
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Adjusting or disabling the processes implemented in simulations can elucidate their impact on observables, but outcomes can be degenerate.
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Similar galaxy populations can emerge in simulations with dissimilar feedback implementations. However, these models generally predict markedly different gas flow rates into, and out of, galaxies and their halos. CGM observations are thus a promising means of breaking this degeneracy and guiding the development of new feedback models
Neutron star mergers and rare core-collapse supernovae as sources of r-process enrichment in simulated galaxies
We use cosmological, magnetohydrodynamical simulations of Milky Way-mass
galaxies from the Auriga project to study their enrichment with rapid neutron
capture (r-process) elements. We implement a variety of enrichment models from
both binary neutron star mergers and rare core-collapse supernovae. We focus on
the abundances of (extremely) metal-poor stars, most of which were formed
during the first ~Gyr of the Universe in external galaxies and later accreted
onto the main galaxy. We find that the majority of metal-poor stars are
r-process enriched in all our enrichment models. Neutron star merger models
result in a median r-process abundance ratio which increases with metallicity,
whereas the median trend in rare core-collapse supernova models is
approximately flat. The scatter in r-process abundance increases for models
with longer delay times or lower rates of r-process producing events. Our
results are nearly perfectly converged, in part due to the mixing of gas
between mesh cells in the simulations. Additionally, different Milky Way-mass
galaxies show only small variation in their respective r-process abundance
ratios. Current (sparse and potentially biased) observations of metal-poor
stars in the Milky Way seem to prefer rare core-collapse supernovae over
neutron star mergers as the dominant source of r-process elements at low
metallicity, but we discuss possible caveats to our models. Dwarf galaxies
which experience a single r-process event early in their history show highly
enhanced r-process abundances at low metallicity, which is seen both in
observations and in our simulations. We also find that the elements produced in
a single event are mixed with ~10^8 Msun of gas relatively quickly,
distributing the r-process elements over a large region.Comment: Accepted for publication in MNRAS. Revised version: added Figure 13
(on mixing of iron and r-process elements) and an Appendix (on iron and
magnesium abundances) and updated the r-process yields (Tables 1 and 2 and
normalization of abundances
Shattering of cosmic sheets due to thermal instabilities: a formation channel for metal-free Lyman limit systems
We present a new cosmological zoom-in simulation, where the zoom region consists of two halos with virial mass M v ~ 5 × 1012 M ⊙ and an approximately megaparsec long cosmic filament connecting them at z ~ 2. Using this simulation, we study the evolution of the intergalactic medium in between these two halos at unprecedented resolution. At 5 gsim z gsim 3, the two halos are found to lie in a large intergalactic sheet, or "pancake," consisting of multiple coplanar dense filaments along which nearly all halos with M v > 109 M ⊙ are located. This sheet collapses at z ~ 5 from the merger of two smaller sheets. The strong shock generated by this merger leads to thermal instabilities in the postshock region, and to a shattering of the sheet resulting in lesssim kiloparsec-scale clouds with temperatures of T gsim 2 × 104 K and densities of n gsim 10−3 cm−3, which are pressure confined in a hot medium with T ~ 106 K and n gsim 10−5 cm−3. When the sheet is viewed face-on, these cold clouds have neutral hydrogen column densities of N H i > 1017.2 cm−2, making them detectable as Lyman limit systems, though they lie well outside the virial radius of any halo and even well outside the dense filaments. Their chemical composition is pristine, having zero metallicity, similar to several recently observed systems. Since these systems form far from any galaxies, these results are robust to galaxy formation physics, resulting purely from the collapse of large-scale structure and radiative cooling, provided sufficient spatial resolution is available
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
Quenching in Cosmic Sheets: Tracing the Impact of Large Scale Structure Collapse on the Evolution of Dwarf Galaxies
Dwarf galaxies are thought to quench primarily due to environmental processes
most typically occurring in galaxy groups and clusters or around single,
massive galaxies. However, at earlier epochs, (), the collapse of
large scale structure (forming Zel'dovich sheets and subsequently filaments of
the cosmic web) can produce volume-filling accretion shocks which elevate large
swaths of the intergalactic medium (IGM) in these structures to a hot (
K) phase. We study the impact of such an event on the evolution of central
dwarf galaxies () in the field using a spatially large,
high resolution cosmological zoom simulation which covers the cosmic web
environment between two protoclusters. We find that the shock-heated sheet acts
as an environmental quencher much like clusters and filaments at lower
redshift, creating a population of quenched, central dwarf galaxies. Even
massive dwarfs which do not quench are affected by the shock, with reductions
to their sSFR and gas accretion. This process can potentially explain the
presence of isolated quenched dwarf galaxies, and represents an avenue of
pre-processing, via which quenched satellites of bound systems quench before
infall.Comment: 15 pages, 10 figures. Submitted to MNRA
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