410 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
Parametrising Star Formation Histories
We examine the star formation histories (SFHs) of galaxies in smoothed
particle hydrodynamics (SPH) simulations, compare them to parametric models
that are commonly used in fitting observed galaxy spectral energy
distributions, and examine the efficacy of these parametric models as practical
tools for recovering the physical parameters of galaxies. The commonly used
tau-model, with SFR ~ exp(-t/tau), provides a poor match to the SFH of our SPH
galaxies, with a mismatch between early and late star formation that leads to
systematic errors in predicting colours and stellar mass-to-light ratios. A
one-parameter lin-exp model, with SFR ~ t*exp(-t/tau), is much more successful
on average, but it fails to match the late-time behavior of the bluest, most
actively star-forming galaxies and the passive, "red and dead" galaxies. We
introduce a 4-parameter model, which transitions from lin-exp to a linear ramp
after a transition time, which describes our simulated galaxies very well. We
test the ability of these parametrised models to recover (at z=0, 0.5, and 1)
the stellar mass-to-light ratios, specific star formation rates, and stellar
population ages from the galaxy colours, computed from the full SPH star
formation histories using the FSPS code of Conroy et al. (2009). Fits with
tau-models systematically overestimate M/L by ~ 0.2 dex, overestimate
population ages by ~ 1-2 Gyr, and underestimate sSFR by ~ 0.05 dex. Fits with
lin-exp are less biased on average, but the 4-parameter model yields the best
results for the full range of galaxies. Marginalizing over the free parameters
of the 4-parameter model leads to slightly larger statistical errors than
1-parameter fits but essentially removes all systematic biases, so this is our
recommended procedure for fitting real galaxies.Comment: 28 pages, 18 figure
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
The Photon Underproduction Crisis
We examine the statistics of the low-redshift Lyman-alpha forest from
smoothed particle hydrodynamic simulations in light of recent improvements in
the estimated evolution of the cosmic ultraviolet background (UVB) and recent
observations from the Cosmic Origins Spectrograph (COS). We find that the value
of the metagalactic photoionization rate required by our simulations to match
the observed properties of the low-redshift Lyman-alpha forest is a factor of 5
larger than the value predicted by state-of-the art models for the evolution of
this quantity. This mismatch results in the mean flux decrement of the
Lyman-alpha forest being underpredicted by at least a factor of 2 (a 10-sigma
discrepancy with observations) and a column density distribution of Lyman-alpha
forest absorbers systematically and significantly elevated compared to
observations over nearly two decades in column density. We examine potential
resolutions to this mismatch and find that either conventional sources of
ionizing photons (galaxies and quasars) must be significantly elevated relative
to current observational estimates or our theoretical understanding of the
low-redshift universe is in need of substantial revision.Comment: Submitted to ApJ Letters; 6 pages including 3 figure
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
Orbital migration of interacting low-mass planets in evolutionary radiative turbulent models
The torques exerted by a locally isothermal disk on an embedded planet lead
to rapid inward migration. Recent work has shown that modeling the
thermodynamics without the assumption of local isothermality reveals regions
where the net torque on an embedded planet is positive, leading to outward
migration of the planet. When a region with negative torque lies directly
exterior to this, planets in the inner region migrate outwards and planets in
the outer region migrate inwards, converging where the torque is zero. We
incorporate the torques from an evolving non-isothermal disk into an N-body
simulation to examine the behavior of planets or planetary embryos interacting
in the convergence zone. We find that mutual interactions do not eject objects
from the convergence zone. Small numbers of objects in a laminar disk settle
into near resonant orbits that remain stable over the 10 Myr periods that we
examine. However, either or both increasing the number of planets or including
a correlated, stochastic force to represent turbulence drives orbit crossings
and mergers in the convergence zone. These processes can build gas giant cores
with masses of order ten Earth masses from sub-Earth mass embryos in 2-3 Myr.Comment: 15 pages, 11 figures. Accepted for publication in Ap
Six rapid assessments of alcohol and other substance use in populations displaced by conflict
BACKGROUND: Substance use among populations displaced by conflict is a neglected area of public health. Alcohol, khat, benzodiazepine, opiate, and other substance use have been documented among a range of displaced populations, with wide-reaching health and social impacts. Changing agendas in humanitarian response-including increased prominence of mental health and chronic illness-have so far failed to be translated into meaningful interventions for substance use. METHODS: Studies were conducted from 2006 to 2008 in six different settings of protracted displacement, three in Africa (Kenya, Liberia, northern Uganda) and three in Asia (Iran, Pakistan, and Thailand). We used intervention-oriented qualitative Rapid Assessment and Response methods, adapted from two decades of experience among non-displaced populations. The main sources of data were individual and group interviews conducted with a culturally representative (non-probabilistic) sample of community members and service providers. RESULTS: Widespread use of alcohol, particularly artisanally-produced alcohol, in Kenya, Liberia, Uganda, and Thailand, and opiates in Iran and Pakistan was believed by participants to be linked to a range of health, social and protection problems, including illness, injury (intentional and unintentional), gender-based violence, risky behaviour for HIV and other sexually transmitted infection and blood-borne virus transmission, as well as detrimental effects to household economy. Displacement experiences, including dispossession, livelihood restriction, hopelessness and uncertain future may make communities particularly vulnerable to substance use and its impact, and changing social norms and networks (including the surrounding population) may result in changed - and potentially more harmful-patterns of use. Limited access to services, including health services, and exclusion from relevant host population programmes, may exacerbate the harmful consequences. CONCLUSIONS: The six studies show the feasibility and value of conducting rapid assessments in displaced populations. One outcome of these studies is the development of a UNHCR/WHO field guide on rapid assessment of alcohol and other substance use among conflict-affected populations. More work is required on gathering population-based epidemiological data, and much more experience is required on delivering effective interventions. Presentation of these findings should contribute to increased awareness, improved response, and more vigorous debate around this important but neglected area
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