1,531 research outputs found
From Galaxy Clusters to Ultra-Faint Dwarf Spheroidals: A Fundamental Curve Connecting Dispersion-supported Galaxies to Their Dark Matter Halos
We examine scaling relations of dispersion-supported galaxies over more than
eight orders of magnitude in luminosity by transforming standard fundamental
plane parameters into a space of mass (M1/2), radius (r1/2), and luminosity
(L1/2). We find that from ultra-faint dwarf spheroidals to giant cluster
spheroids, dispersion-supported galaxies scatter about a one-dimensional
"fundamental curve" through this MRL space. The weakness of the M1/2-L1/2 slope
on the faint end may imply that potential well depth limits galaxy formation in
small galaxies, while the stronger dependence on L1/2 on the bright end
suggests that baryonic physics limits galaxy formation in massive galaxies. The
mass-radius projection of this curve can be compared to median dark matter halo
mass profiles of LCDM halos in order to construct a virial mass-luminosity
relationship (Mvir-L) for galaxies that spans seven orders of magnitude in
Mvir. Independent of any global abundance or clustering information, we find
that (spheroidal) galaxy formation needs to be most efficient in halos of Mvir
~ 10^12 Msun and to become inefficient above and below this scale. Moreover,
this profile matching technique is most accurate at the high and low luminosity
extremes (where dark matter fractions are highest) and is therefore quite
complementary to statistical approaches that rely on having a well-sampled
luminosity function. We also consider the significance and utility of the
scatter about this relation, and find that in the dSph regime observational
errors are almost at the point where we can explore the intrinsic scatter in
the luminosity-virial mass relation. Finally, we note that purely stellar
systems like Globular Clusters and Ultra Compact Dwarfs do not follow the
fundamental curve relation. This allows them to be easily distinguished from
dark-matter dominated dSph galaxies in MRL space. (abridged)Comment: 27 pages, 18 figures, ApJ accepted. High-res movies of 3D figures are
available at http://www.physics.uci.edu/~bullock/fcurve/movies.htm
BDM Dark Matter: CDM with a core profile and a free streaming scale
We present a new dark matter model BDM which is an hybrid between hot dark
matter HDM and cold dark matter CDM, in which the BDM particles behave as HDM
above the energy scale E_c and as CDM below this scale. Evolution of structure
formation is similar to that of CDM model but BDM predicts a nonvanishing free
streaming l_fs scale and a inner galaxy core radius r_core, both quantities
determined in terms of a single parameter E_c, which corresponds to the phase
transition energy scale of the subjacent elementary particle model. For
energies above E_c or for a scale factor a smaller then a_c, with a<a_c<a_{eq},
the particles are massless and rho redshifts as radiation. However, once the
energy becomes E\leq E_c or a>a_c then the BDM particles acquire a large mass
through a non perturbative mechanism, as baryons do, and rho redshifts as
matter with the particles having a vanishing velocity. Typical energies are
E_c=O(10-100) eV giving a l_fs \propto E_c^{-4/3}\lesssim Mpc and m_fs\propto
E_c^{-4}\lesssim 10^9 M\odot. A l_fs\neq 0, r_core\neq 0 help to resolve some
of the shortcomings of CDM such as overabundance substructure in CDM halos and
numerical fit to rotation curves in dwarf spheroidal and LSB galaxies. Finally,
our BDM model and the phase transition scale E_c can be derived from particle
physics.Comment: 7 pages, 8 figure
Cosmological Simulations with Self-Interacting Dark Matter I: Constant Density Cores and Substructure
We use cosmological simulations to study the effects of self-interacting dark
matter (SIDM) on the density profiles and substructure counts of dark matter
halos from the scales of spiral galaxies to galaxy clusters, focusing
explicitly on models with cross sections over dark matter particle mass
\sigma/m = 1 and 0.1 cm^2/g. Our simulations rely on a new SIDM N-body
algorithm that is derived self-consistently from the Boltzmann equation and
that reproduces analytic expectations in controlled numerical experiments. We
find that well-resolved SIDM halos have constant-density cores, with
significantly lower central densities than their CDM counterparts. In contrast,
the subhalo content of SIDM halos is only modestly reduced compared to CDM,
with the suppression greatest for large hosts and small halo-centric distances.
Moreover, the large-scale clustering and halo circular velocity functions in
SIDM are effectively identical to CDM, meaning that all of the large-scale
successes of CDM are equally well matched by SIDM. From our largest cross
section runs we are able to extract scaling relations for core sizes and
central densities over a range of halo sizes and find a strong correlation
between the core radius of an SIDM halo and the NFW scale radius of its CDM
counterpart. We construct a simple analytic model, based on CDM scaling
relations, that captures all aspects of the scaling relations for SIDM halos.
Our results show that halo core densities in \sigma/m = 1 cm^2/g models are too
low to match observations of galaxy clusters, low surface brightness spirals
(LSBs), and dwarf spheroidal galaxies. However, SIDM with \sigma/m ~ 0.1 cm^2/g
appears capable of reproducing reported core sizes and central densities of
dwarfs, LSBs, and galaxy clusters without the need for velocity dependence.
(abridged)Comment: 26 pages, 16 figures, all figures include colors, submitted for
publication in MNRA
An HST/ACS investigation of the spatial and chemical structure and sub-structure of NGC 891, a Milky Way analogue
We present a structural analysis of NGC891, an edge-on galaxy that has long
been considered to be an analogue of the Milky Way. Using starcounts derived
from deep HST/ACS images, we detect the presence of a thick disk component in
this galaxy with vertical scale height 1.44+/-0.03 kpc and radial scale length
4.8+/-0.1 kpc, only slightly longer than that of the thin disk. A stellar
spheroid with a de Vaucouleurs-like profile is detected from a radial distance
of 0.5 kpc to the edge of the survey at 25 kpc; the structure appears to become
more flattened with distance, reaching q = 0.50 in the outermost halo region
probed. The halo inside of 15 kpc is moderately metal-rich (median [Fe/H] ~
-1.1) and approximately uniform in median metallicity. Beyond that distance a
modest chemical gradient is detected, with the median reaching [Fe/H] ~ -1.3 at
20 kpc. We find evidence for subtle, but very significant, small-scale
variations in the median colour and density over the halo survey area. We argue
that the colour variations are unlikely to be due to internal extinction or
foreground extinction, and reflect instead variations in the stellar
metallicity. Their presence suggests a startling conclusion: that the halo of
this galaxy is composed of a large number of incompletely-mixed
sub-populations, testifying to its origin in a deluge of small accretions.Comment: 21 pages, 16 figures, accepted for publication in MNRA
LoCuSS: Connecting the Dominance and Shape of Brightest Cluster Galaxies with the Assembly History of Massive Clusters
We study the luminosity gap, dm12, between the first and second ranked
galaxies in a sample of 59 massive galaxy clusters, using data from the Hale
Telescope, HST, Chandra, and Spitzer. We find that the dm12 distribution,
p(dm12), is a declining function of dm12, to which we fitted a straight line:
p(dm12) propto -(0.13+/-0.02)dm12. The fraction of clusters with "large"
luminosity gaps is p(dm12>=1)=0.37+/-0.08, which represents a 3sigma excess
over that obtained from Monte Carlo simulations of a Schechter function that
matches the mean cluster galaxy luminosity function. We also identify four
clusters with "extreme" luminosity gaps, dm12>=2, giving a fraction of
p(dm12>=2)=0.07+0.05-0.03. More generally, large luminosity gap clusters are
relatively homogeneous, with elliptical/disky brightest cluster galaxies
(BCGs), cuspy gas density profiles (i.e. strong cool cores), high
concentrations, and low substructure fractions. In contrast, small luminosity
gap clusters are heterogeneous, spanning the full range of
boxy/elliptical/disky BCG morphologies, the full range of cool core strengths
and dark matter concentrations, and have large substructure fractions. Taken
together, these results imply that the amplitude of the luminosity gap is a
function of both the formation epoch, and the recent infall history of the
cluster. "BCG dominance" is therefore a phase that a cluster may evolve
through, and is not an evolutionary "cul-de-sac". We also compare our results
with semi-analytic model predictions based on the Millennium Simulation. None
of the models are able to reproduce all of the observational results,
underlining the inability of current models to match the empirical properties
of BCGs. We identify the strength of AGN feedback and the efficiency with which
cluster galaxies are replenished after they merge with the BCG in each model as
possible causes of these discrepancies. [Abridged]Comment: 15 pages, 12 figures, accepted for publication in MNRA
Modelling non-linear redshift-space distortions in the galaxy clustering pattern: systematic errors on the growth rate parameter
We investigate the ability of state-of-the-art redshift-space distortions
models for the galaxy anisotropic two-point correlation function \xi(r_p, \pi),
to recover precise and unbiased estimates of the linear growth rate of
structure f, when applied to catalogues of galaxies characterised by a
realistic bias relation. To this aim, we make use of a set of simulated
catalogues at z = 0.1 and z = 1 with different luminosity thresholds, obtained
by populating dark-matter haloes from a large N-body simulation using halo
occupation prescriptions. We examine the most recent developments in
redshift-space distortions modelling, which account for non-linearities on both
small and intermediate scales produced respectively by randomised motions in
virialised structures and non-linear coupling between the density and velocity
fields. We consider the possibility of including the linear component of galaxy
bias as a free parameter and directly estimate the growth rate of structure f.
Results are compared to those obtained using the standard dispersion model,
over different ranges of scales.We find that the model of Taruya et al. (2010),
the most sophisticated one considered in this analysis, provides in general the
most unbiased estimates of the growth rate of structure, with systematic errors
within 4% over a wide range of galaxy populations spanning luminosities between
L > L* and L > 3L*. The scale-dependence of galaxy bias plays a role on
recovering unbiased estimates of f when fitting quasi non-linear scales. Its
effect is particularly severe for most luminous galaxies, for which systematic
effects in the modelling might be more difficult to mitigate and have to be
further investigated. [...]Comment: 17 pages, 16 figures, Accepted for publication in MNRA
The Luminosity Function of the NoSOCS Galaxy Cluster Sample
We present the analysis of the luminosity function of a large sample of
galaxy clusters from the Northern Sky Optical Cluster Survey, using latest data
from the Sloan Digital Sky Survey. Our global luminosity function (down to
M_r<= -16) does not show the presence of an upturn at faint magnitudes, while
we do observe a strong dependence of its shape on both richness and
cluster-centric radius, with a brightening of M^* and an increase of the dwarf
to giant ratio with richness, indicating that more massive systems are more
efficient in creating/retaining a population of dwarf satellites. This is
observed both within physical (0.5 R_200) and fixed (0.5 Mpc) apertures,
suggesting that the trend is either due to a global effect, operating at all
scales, or to a local one but operating on even smaller scales. We further
observe a decrease of the relative number of dwarf galaxies towards the cluster
center; this is most probably due to tidal collisions or collisional disruption
of the dwarfs since merging processes are inhibited by the high velocity
dispersions in cluster cores and, furthermore, we do not observe a strong
dependence of the bright end on the environment. We find indication that the
dwarf to giant ratio decreases with increasing redshift, within 0.07<z<0.2. We
also measure a trend for stronger suppression of faint galaxies (below M^*+2)
with increasing redshift in poor systems, with respect to more massive ones,
indicating that the evolutionary stage of less massive galaxies depends more
critically on the environment. Finally we point out that the luminosity
function is far from universal; hence the uncertainties introduced by the
different methods used to build a composite function may partially explain the
variety of faint-end slopes reported in the literature as well as, in some
cases, the presence of a faint-end upturn.Comment: 16 pages, 12 figures. Accepted for publication in MNRA
Reading faces: differential lateral gaze bias in processing canine and human facial expressions in dogs and 4-year-old children
Sensitivity to the emotions of others provides clear biological advantages. However, in the case of heterospecific relationships, such as that existing between dogs and humans, there are additional challenges since some elements of the expression of emotions are species-specific. Given that faces provide important visual cues for communicating emotional state in both humans and dogs, and that processing of emotions is subject to brain lateralisation, we investigated lateral gaze bias in adult dogs when presented with pictures of expressive human and dog faces. Our analysis revealed clear differences in laterality of eye movements in dogs towards conspecific faces according to the emotional valence of the expressions. Differences were also found towards human faces, but to a lesser extent. For comparative purpose, a similar experiment was also run with 4-year-old children and it was observed that they showed differential processing of facial expressions compared to dogs, suggesting a species-dependent engagement of the right or left hemisphere in processing emotions
A pragmatic cluster randomised trial evaluating three implementation interventions
Background
Implementation research is concerned with bridging the gap between evidence and practice through the study of methods to promote the uptake of research into routine practice. Good quality evidence has been summarised into guideline recommendations to show that peri-operative fasting times could be considerably shorter than patients currently experience. The objective of this trial was to evaluate the effectiveness of three strategies for the implementation of recommendations about peri-operative fasting.
Methods
A pragmatic cluster randomised trial underpinned by the PARIHS framework was conducted during 2006 to 2009 with a national sample of UK hospitals using time series with mixed methods process evaluation and cost analysis. Hospitals were randomised to one of three interventions: standard dissemination (SD) of a guideline package, SD plus a web-based resource championed by an opinion leader, and SD plus plan-do-study-act (PDSA). The primary outcome was duration of fluid fast prior to induction of anaesthesia. Secondary outcomes included duration of food fast, patients' experiences, and stakeholders' experiences of implementation, including influences. ANOVA was used to test differences over time and interventions.
Results
Nineteen acute NHS hospitals participated. Across timepoints, 3,505 duration of fasting observations were recorded. No significant effect of the interventions was observed for either fluid or food fasting times. The effect size was 0.33 for the web-based intervention compared to SD alone for the change in fluid fasting and was 0.12 for PDSA compared to SD alone. The process evaluation showed different types of impact, including changes to practices, policies, and attitudes. A rich picture of the implementation challenges emerged, including inter-professional tensions and a lack of clarity for decision-making authority and responsibility.
Conclusions
This was a large, complex study and one of the first national randomised controlled trials conducted within acute care in implementation research. The evidence base for fasting practice was accepted by those participating in this study and the messages from it simple; however, implementation and practical challenges influenced the interventions' impact. A set of conditions for implementation emerges from the findings of this study, which are presented as theoretically transferable propositions that have international relevance. Trial registration ISRCTN18046709 - Peri-operative Implementation Study Evaluation (POISE
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