2,321 research outputs found
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
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
The Rise and Fall of Galaxy Activity in Dark Matter Haloes
We use a SDSS galaxy group catalogue to study the dependence of galaxy
activity on stellar mass, halo mass, and group hierarchy (centrals vs.
satellites). We split our galaxy sample in star-forming galaxies, galaxies with
optical AGN activity and radio sources. We find a smooth transition in halo
mass as the activity of central galaxies changes from star formation to optical
AGN activity to radio emission. Star-forming centrals preferentially reside in
haloes with M<10^{12} Msun, central galaxies with optical-AGN activity
typically inhabit haloes with M \sim 10^{13} Msun, and centrals emitting in the
radio mainly reside in haloes more massive than 10^{14} Msun. Although this
seems to suggest that the environment (halo mass) determines the type of
activity of its central galaxy, we find a similar trend with stellar mass:
central star formers typically have stellar masses below 10^{10} Msun, while
optical-AGN hosts and central radio sources have characteristic stellar masses
of 10^{10.8} Msun and 10^{11.6} Msun, respectively. Since more massive haloes
typically host more massive centrals, it is unclear whether the activity of a
central galaxy is causally connected to its stellar mass or to its halo mass.
In general, satellite galaxies have their activity suppressed wrt central
galaxies of the same stellar mass. At fixed stellar mass, we find that the
activity of satellite galaxies depends only weakly on halo mass. In fact, for
satellite galaxies the dependence of galaxy activity on halo mass is more than
four times weaker than the dependence on stellar mass. As we discuss, all these
results are consistent with a picture in which low mass haloes accrete cold
gas, while massive haloes have coronae of hot gas that promote radio activity
of their central galaxies. [Abridged]Comment: 17 pages, 13 figures. Submitted for publication in MNRA
Feedback from galactic stellar bulges and hot gaseous haloes of galaxies
We demonstrate that the feedback from stellar bulges can play an essential
role in shaping the halo gas of galaxies with substantial bulge components by
conducting 1-D hydrodynamical simulations. The feedback model we consider
consists of two distinct phases: 1) an early starburst during the bulge
formation and 2) a subsequent long-lasting mass and energy injection from
stellar winds of low-mass stars and Type Ia SNe. An energetic outward blastwave
is initiated by the starburst and is maintained and enhanced by the
long-lasting stellar feedback. For a MW-like galactic bulge, this blastwave
sweeps up the halo gas in the proto-galaxy and heats up the surrounding medium
to a scale much beyond the virial radius of the halo, thus the accretion of the
halo hot gas can be completely stopped. In addition, the long-lasting feedback
in the later phase powers a galactic bulge wind that is reverse-shocked at a
large radius in the presence of surrounding intergalactic medium and hence
maintains a hot gaseous halo. As the mass and energy injection decreases with
time, the feedback evolves to a subsonic and quasi-stable outflow, which is
enough to prevent halo gas from cooling. The two phases of the feedback thus
re-enforce each-other's impact on the gas dynamics. The simulation results
demonstrate that the stellar bulge feedback may provide a plausible solution to
the long-standing problems in understanding the MW type galaxies, such as the
"missing stellar feedback" problem and the "over-cooling" problem. The
simulations also show that the properties of the hot gas in the subsonic
outflow state depend sensitively on the environment and the formation history
of the bulge. This dependence and variance may explain the large dispersion in
the X-ray to B-band luminosity ratio of the low Es.Comment: v2, discussions added, accepted for publication in MNRA
Reporting interventions in trials evaluating cognitive rehabilitation in people with Multiple Sclerosis: a systematic review
Objective: To determine the quantity and quality of description of cognitive rehabilitation for cognitive deficits in people with Multiple Sclerosis, using a variety of published checklists, and suggest ways of improving the reporting of these interventions.
Data sources: Ten electronic databases were searched, including MEDLINE, EMBASE, CINAHL and PsycINFO, from inception to May 2017. Grey literature databases, trials registers, reference lists and author citations were also searched.
Review methods: Papers were included if participants were people with multiple sclerosis aged 18 years and over, and if the effectiveness of cognitive rehabilitation in improving functional ability for memory, attention or executive dysfunction, with or without a control group, was being evaluated.
Results: Fifty-four studies were included in this review. The reporting of a number of key aspects of cognitive rehabilitation was poor. This was particularly in relation to content of interventions (reported completely in 26 of the 54 studies), intervention procedures (reported completely in 16 of the 54 studies), delivery mode (reported completely in 24 of the 54 studies) and intervention mechanism of action (reported completely in 21 of the 54 studies).
Conclusion: The quality of reporting of cognitive rehabilitation for memory, attention and executive function for multiple sclerosis, across a range of study designs, is poor. Existing reporting checklists do not adequately cover aspects relevant to cognitive rehabilitation, such as the approaches used to address cognitive deficits. Future checklists could consider these aspects we have identified in this review
Galaxies in a Simulated CDM Universe II: Observable Properties and Constraints on Feedback
We compare the properties of galaxies that form in a cosmological simulation
without strong feedback to observations at z=0. We confirm previous findings
that models without strong feedback overproduce the observed galaxy baryonic
mass function, especially at the low and high mass extremes. Through
post-processing we investigate what kinds of feedback would be required to
reproduce observed galaxy masses and star formation rates. To mimic an extreme
form of "preventive" feedback (e.g., AGN radio mode) we remove all baryonic
mass that was originally accreted via "hot mode" from shock-heated gas. This
does not bring the high mass end of the galaxy mass function into agreement
with observations because much of the stellar mass in these systems formed at
high redshift from baryons that originally accreted via "cold mode" onto lower
mass progenitors. An efficient "ejective" feedback mechanism, such as supernova
driven winds, must reduce the masses of these progenitors. Feedback must also
reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts
and have much lower star formation rates. If we monotonically re-map galaxy
masses to reproduce the observed mass function, but retain the simulation's
predicted star formation rates, we obtain fairly good agreement with the
observed sequence of star-forming galaxies but fail to recover the observed
population of passive, low star formation rate galaxies. Suppressing all hot
mode accretion improves agreement for high mass galaxies but worsens the
agreement at intermediate masses. Reproducing these z=0 observations requires a
feedback mechanism that dramatically suppresses star formation in a fraction of
galaxies, increasing with mass, while leaving star formation rates of other
galaxies essentially unchanged.Comment: MNRAS in press. 15 pages, 5 figures, minimal changes from the first
versio
Making Galaxies in a Cosmological Context: The Need for Early Stellar Feedback
We introduce the Making Galaxies in a Cosmological Context (MaGICC) program
of smoothed particle hydrodynamics (SPH) simulations. We describe a parameter
study of galaxy formation simulations of an L* galaxy that uses early stellar
feedback combined with supernova feedback to match the stellar mass--halo mass
relationship. While supernova feedback alone can reduce star formation enough
to match the stellar mass--halo mass relationship, the galaxy forms too many
stars before z=2 to match the evolution seen using abundance matching. Our
early stellar feedback is purely thermal and thus operates like a UV ionization
source as well as providing some additional pressure from the radiation of
massive, young stars. The early feedback heats gas to >10^6 K before cooling to
10^4 K. The pressure from this hot gas creates a more extended disk and
prevents more star formation prior to z=1 than supernovae feedback alone. The
resulting disk galaxy has a flat rotation curve, an exponential surface
brightness profile, and matches a wide range of disk scaling relationships. The
disk forms from the inside-out with an increasing exponential scale length as
the galaxy evolves. Overall, early stellar feedback helps to simulate galaxies
that match observational results at low and high redshifts.Comment: 13 pages, 14 figures, accepted MNRAS, movies at
http://www.mpia.de/~stinson/magic
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