77 research outputs found
Recoiling Supermassive Black Hole Escape Velocities from Dark Matter Halos
We simulate recoiling black hole trajectories from to in dark
matter halos, quantifying how parameter choices affect escape velocities. These
choices include the strength of dynamical friction, the presence of stars and
gas, the accelerating expansion of the universe (Hubble acceleration), host
halo accretion and motion, and seed black hole mass. CDM halo
accretion increases escape velocities by up to 0.6 dex and significantly
shortens return timescales compared to non-accreting cases. Other parameters
change orbit damping rates but have subdominant effects on escape velocities;
dynamical friction is weak at halo escape velocities, even for extreme
parameter values. We present formulae for black hole escape velocities as a
function of host halo mass and redshift. Finally, we discuss how these findings
affect black hole mass assembly as well as minimum stellar and halo masses
necessary to retain supermassive black holes.Comment: 10 pages, 17 figures. Updated to correct a typo (sign error) in fit
to escape velocity, for return by z=0 (eq. 19
Stellar Disks in Aquarius Dark Matter Haloes
We investigate the gravitational interactions between live stellar disks and
their dark matter halos, using LCDM haloes similar in mass to that of the Milky
Way taken from the Aquarius Project. We introduce the stellar disks by first
allowing the haloes to respond to the influence of a growing rigid disk
potential from z = 1.3 to z = 1.0. The rigid potential is then replaced with
star particles which evolve self-consistently with the dark matter particles
until z = 0.0. Regardless of the initial orientation of the disk, the inner
parts of the haloes contract and change from prolate to oblate as the disk
grows to its full size. When the disk normal is initially aligned with the
major axis of the halo at z=1.3, the length of the major axis contracts and
becomes the minor axis by z=1.0. Six out of the eight disks in our main set of
simulations form bars, and five of the six bars experience a buckling
instability that results in a sudden jump in the vertical stellar velocity
dispersion and an accompanying drop in the m=2 Fourier amplitude of the disk
surface density. The bars are not destroyed by the buckling but continue to
grow until the present day. Bars are largely absent when the disk mass is
reduced by a factor of two or more; the relative disk-to-halo mass is therefore
a primary factor in bar formation and evolution. A subset of the disks is
warped at the outskirts and contains prominent non-coplanar material with a
ring-like structure. Many disks reorient by large angles between z=1 and z=0,
following a coherent reorientation of their inner haloes. Larger reorientations
produce more strongly warped disks, suggesting a tight link between the two
phenomena. The origins of bars and warps appear independent: some disks with
strong bars show no disturbances at the outskirts, while the disks with the
weakest bars show severe warps.Comment: 19 pages, 13 figures, accepted MNRAS; fixed compatibility problem in
figures 8,
Galaxy clustering in the NEWFIRM Medium Band Survey: the relationship between stellar mass and dark matter halo mass at 1 < z < 2
We present an analysis of the clustering of galaxies as a function of their
stellar mass at 1 < z < 2 using data from the NEWFIRM Medium Band Survey
(NMBS). The precise photometric redshifts and stellar masses that the NMBS
produces allows us to define a series of mass limited samples of galaxies more
massive than 0.7, 1 and 3x10^10 Msun in redshift intervals centered on z = 1.1,
1.5 and 1.9 respectively. In each redshift interval we show that there exists a
strong dependence of clustering strength on the stellar mass limit of the
sample, with more massive galaxies showing a higher clustering amplitude on all
scales. We further interpret our clustering measurements in the LCDM
cosmological context using the halo model of galaxy clustering. We show that
the typical halo mass of central and satellite galaxies increases with stellar
mass, whereas the satellite fraction decreases with stellar mass, qualitatively
the same as is seen at z < 1. We see little evidence of any redshift dependence
in the stellar mass-to-halo mass relationship over our narrow redshift range.
However, when we compare with similar measurements at z~0, we see clear
evidence for a change in this relation. If we assume a universal baryon
fraction, the ratio of stellar mass to halo mass reveals the fraction of
baryons that have been converted to stars. We see that the peak in this star
formation efficiency for central galaxies shifts to higher halo masses at
higher redshift, moving from ~7x10^11 Msun at z~0 to ~3x10^12 Msun at z~1.5,
revealing evidence of `halo downsizing'. Finally we show that for highly biased
galaxy populations at z > 1 there may be a discrepancy between the measured
space density and clustering and that predicted by the halo model. This could
imply that there is a problem with one or more ingredients of the halo model at
these redshifts, for instance the halo bias relation or the halo profile.Comment: Accepted for publication in ApJ. Correction made to typo in halo
masses in conclusion
Dissecting the roles of mass and environment quenching in galaxy evolution with EAGLE
We exploit the pioneering cosmological hydrodynamical simulation, EAGLE, to
study how the connection between halo mass (M_halo), stellar mass (M*) and
star-formation rate (SFR) evolves across redshift. Using Principal Component
Analysis we identify the key axes of correlation between these physical
quantities, for the full galaxy sample and split by satellite/central and
low/high halo mass. The first principal component of the z=0 EAGLE galaxy
population is a positive correlation between M_halo, M* and SFR. This component
is particularly dominant for central galaxies in low mass haloes. The second
principal component, most significant in high mass haloes, is a negative
correlation between M_halo and SFR, indicative of environmental quenching. For
galaxies above M*~10^10M_solar, however, the SFR is seen to decouple from the
M_halo-M* correlation; this result is found to be independent of environment,
suggesting that mass quenching effects are also in operation. We find extremely
good agreement between the EAGLE principal components and those of SDSS
galaxies; this lends confidence to our conclusions. Extending our study to
EAGLE galaxies in the range z=0-4, we find that, although the relative numbers
of galaxies in the different subsamples change, their principal components do
not change significantly with redshift. This indicates that the physical
processes that govern the evolution of galaxies within their dark matter haloes
act similarly throughout cosmic time. Finally, we present halo occupation
distribution model fits to EAGLE galaxies and show that one flexible
6-parameter functional form is capable of fitting a wide range of different
mass- and SFR-selected subsamples.Comment: 17 pages, 5 figures; accepted for publication in MNRA
Satellite abundances around bright isolated galaxies
We study satellite galaxy abundances in SDSS by counting photometric galaxies
around isolated bright primaries. We present results as a function of the
luminosity, stellar mass and colour of the satellites, and of the stellar mass
and colour of the primaries. For massive primaries the luminosity and stellar
mass functions of satellites are similar in shape to those of field galaxies,
but for lower mass primaries they are significantly steeper. The steepening is
particularly marked for the stellar mass function. Satellite abundance
increases strongly with primary stellar mass, approximately in proportion to
expected dark halo mass. Massive red primaries have up to a factor of 2 more
satellites than blue ones of the same stellar mass. Satellite galaxies are
systematically redder than field galaxies of the same stellar mass. Satellites
are also systematically redder around more massive primaries. At fixed primary
mass, they are redder around red primaries. We select similarly isolated
galaxies from mock catalogues based on the simulations of Guo et al.(2011) and
analyze them in parallel with the SDSS data. The simulation reproduces all the
above trends qualitatively, except for the steepening of the satellite
luminosity and stellar mass functions. Model satellites, however, are
systematically redder than in the SDSS, particularly at low mass and around
low-mass primaries. Simulated haloes of a given mass have satellite abundances
that are independent of central galaxy colour, but red centrals tend to have
lower stellar masses, reflecting earlier quenching of their star formation by
feedback. This explains the correlation between satellite abundance and primary
colour in the simulation. The correlation between satellite colour and primary
colour arises because red centrals live in haloes which are more massive, older
and more gas-rich, so that satellite quenching is more efficient.Comment: 29 pages, 24 figure
Cosmological Constraints from a Combination of Galaxy Clustering and Lensing -- III. Application to SDSS Data
We simultaneously constrain cosmology and galaxy bias using measurements of
galaxy abundances, galaxy clustering and galaxy-galaxy lensing taken from the
Sloan Digital Sky Survey. We use the conditional luminosity function (which
describes the halo occupation statistics as function of galaxy luminosity)
combined with the halo model (which describes the non-linear matter field in
terms of its halo building blocks) to describe the galaxy-dark matter
connection. We explicitly account for residual redshift space distortions in
the projected galaxy-galaxy correlation functions, and marginalize over
uncertainties in the scale dependence of the halo bias and the detailed
structure of dark matter haloes. Under the assumption of a spatially flat,
vanilla {\Lambda}CDM cosmology, we focus on constraining the matter density,
{\Omega}m, and the normalization of the matter power spectrum, {\sigma}8, and
we adopt WMAP7 priors for the spectral index, the Hubble parameter, and the
baryon density. We obtain that \Omegam = 0.278_{-0.026}^{+0.023} and {\sigma}8
= 0.763_{-0.049}^{+0.064} (95% CL). These results are robust to uncertainties
in the radial number density distribution of satellite galaxies, while allowing
for non-Poisson satellite occupation distributions results in a slightly lower
value for {\sigma}8 (0.744_{-0.047}^{+0.056}). These constraints are in
excellent agreement (at the 1{\sigma} level) with the cosmic microwave
background constraints from WMAP. This demonstrates that the use of a realistic
and accurate model for galaxy bias, down to the smallest non-linear scales
currently observed in galaxy surveys, leads to results perfectly consistent
with the vanilla {\Lambda}CDM cosmology.Comment: 21 pages, 9 figures, 5 tables, submitted to MNRA
Gas Accretion and Galactic Chemical Evolution: Theory and Observations
This chapter reviews how galactic inflows influence galaxy metallicity. The
goal is to discuss predictions from theoretical models, but particular emphasis
is placed on the insights that result from using models to interpret
observations. Even as the classical G-dwarf problem endures in the latest round
of observational confirmation, a rich and tantalizing new phenomenology of
relationships between , , SFR, and gas fraction is emerging both in
observations and in theoretical models. A consensus interpretation is emerging
in which star-forming galaxies do most of their growing in a quiescent way that
balances gas inflows and gas processing, and metal dilution with enrichment.
Models that explicitly invoke this idea via equilibrium conditions can be used
to infer inflow rates from observations, while models that do not assume
equilibrium growth tend to recover it self-consistently. Mergers are an overall
subdominant mechanism for delivering fresh gas to galaxies, but they trigger
radial flows of previously-accreted gas that flatten radial gas-phase
metallicity gradients and temporarily suppress central metallicities. Radial
gradients are generically expected to be steep at early times and then
flattened by mergers and enriched inflows of recycled gas at late times.
However, further theoretical work is required in order to understand how to
interpret observations. Likewise, more observational work is needed in order to
understand how metallicity gradients evolve to high redshifts.Comment: Invited review to appear in Gas Accretion onto Galaxies, Astrophysics
and Space Science Library, eds. A. J. Fox & R. Dav\'e, to be published by
Springer. 29 pages, 2 figure
Spontaneous Abortion and Preterm Labor and Delivery in Nonhuman Primates: Evidence from a Captive Colony of Chimpanzees (Pan troglodytes)
Preterm birth is a leading cause of perinatal mortality, yet the evolutionary history of this obstetrical syndrome is largely unknown in nonhuman primate species.We examined the length of gestation during pregnancies that occurred in a captive chimpanzee colony by inspecting veterinary and behavioral records spanning a total of thirty years. Upon examination of these records we were able to confidently estimate gestation length for 93 of the 97 (96%) pregnancies recorded at the colony. In total, 78 singleton gestations resulted in live birth, and from these pregnancies we estimated the mean gestation length of normal chimpanzee pregnancies to be 228 days, a finding consistent with other published reports. We also calculated that the range of gestation in normal chimpanzee pregnancies is approximately forty days. Of the remaining fifteen pregnancies, only one of the offspring survived, suggesting viability for chimpanzees requires a gestation of approximately 200 days. These fifteen pregnancies constitute spontaneous abortions and preterm deliveries, for which the upper gestational age limit was defined as 2 SD from the mean length of gestation (208 days).The present study documents that preterm birth occurred within our study population of captive chimpanzees. As in humans, pregnancy loss is not uncommon in chimpanzees, In addition, our findings indicate that both humans and chimpanzees show a similar range of normal variation in gestation length, suggesting this was the case at the time of their last common ancestor (LCA). Nevertheless, our data suggest that whereas chimpanzees' normal gestation length is ∼20-30 days after reaching viability, humans' normal gestation length is approximately 50 days beyond the estimated date of viability without medical intervention. Future research using a comparative evolutionary framework should help to clarify the extent to which mechanisms at work in normal and preterm parturition are shared in these species
Mergers in Lambda-CDM: Uncertainties in Theoretical Predictions and Interpretations of the Merger Rate
Different methodologies lead to order-of-magnitude variations in predicted
galaxy merger rates. We examine and quantify the dominant uncertainties.
Different halo merger rates and subhalo 'destruction' rates agree to within a
factor ~2 given proper care in definitions. If however (sub)halo masses are not
appropriately defined or are under-resolved, the major merger rate can be
dramatically suppressed. The dominant differences in galaxy merger rates owe to
baryonic physics. Hydrodynamic simulations without feedback and older models
that do not agree with the observed galaxy mass function propagate factor ~5
bias in the resulting merger rates. However, if the model matches the galaxy
mass function, properties of central galaxies are sufficiently converged to
give small differences in merger rates. But variations in baryonic physics of
satellites also have dramatic effects. The known problem of satellite
'over-quenching' in most semi-analytic models (SAMs), whereby SAM satellites
are too efficiently stripped of gas, could lead to order-of-magnitude
under-estimates of merger rates for low-mass, gas-rich galaxies. Fixing the
satellite properties to observations tends to predict higher merger rates, but
with factor ~2 empirical uncertainties. Choice of mass ratio definition
matters: at low masses, most true major mergers (in baryonic/dynamical galaxy
mass) will appear to be minor mergers in their stellar or luminosity mass
ratio. Observations and models using these criteria may underestimate major
merger rates by factors ~5. Orbital parameters and gas fractions also introduce
factor ~3 differences in amount of bulge formed by mergers, even for fixed mass
ratio encounters.Comment: 32 Pages, 15 figures, accepted to ApJ (revised to match accepted
version and correct Fig. 12
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