1,015 research outputs found
Galaxy-Induced Transformation of Dark Matter Halos
We use N-body/gasdynamical LambdaCDM cosmological simulations to examine the
effect of the assembly of a central galaxy on the shape and mass profile of its
dark halo. Two series of simulations are compared; one that follows only the
evolution of the dark matter component and a second one where a baryonic
component is added. These simulations include radiative cooling but neglect
star formation and feedback, leading most baryons to collect at the halo center
in a disk which is too small and too massive when compared with typical spiral.
This unrealistic model allows us, nevertheless, to gauge the maximum effect
that galaxies may have in transforming their dark halos. We find that the shape
of the halo becomes more axisymmetric: halos are transformed from triaxial into
essentially oblate systems, with well-aligned isopotential contours of roughly
constant flattening (c/a ~ 0.85). Halos always contract as a result of galaxy
assembly, but the effect is substantially less pronounced than predicted by the
"adiabatic contraction" hypothesis. The reduced contraction helps to reconcile
LambdaCDM halos with constraints on the dark matter content inside the solar
circle and should alleviate the long-standing difficulty of matching
simultaneously the scaling properties of galaxy disks and the luminosity
function. The halo contraction is also less pronounced than found in earlier
simulations, a disagreement that suggests that halo contraction is not solely a
function of the initial and final distribution of baryons. Not only how much
baryonic mass has been deposited at the center of a halo matters, but also the
mode of its deposition. It might prove impossible to predict the halo response
without a detailed understanding of a galaxy's assembly history. (Abriged)Comment: 11 pages and 9 figure
Laser frequency comb techniques for precise astronomical spectroscopy
Precise astronomical spectroscopic analyses routinely assume that individual
pixels in charge-coupled devices (CCDs) have uniform sensitivity to photons.
Intra-pixel sensitivity (IPS) variations may already cause small systematic
errors in, for example, studies of extra-solar planets via stellar radial
velocities and cosmological variability in fundamental constants via quasar
spectroscopy, but future experiments requiring velocity precisions approaching
~1 cm/s will be more strongly affected. Laser frequency combs have been shown
to provide highly precise wavelength calibration for astronomical
spectrographs, but here we show that they can also be used to measure IPS
variations in astronomical CCDs in situ. We successfully tested a laser
frequency comb system on the Ultra-High Resolution Facility spectrograph at the
Anglo-Australian Telescope. By modelling the 2-dimensional comb signal recorded
in a single CCD exposure, we find that the average IPS deviates by <8 per cent
if it is assumed to vary symmetrically about the pixel centre. We also
demonstrate that series of comb exposures with absolutely known offsets between
them can yield tighter constraints on symmetric IPS variations from ~100
pixels. We discuss measurement of asymmetric IPS variations and absolute
wavelength calibration of astronomical spectrographs and CCDs using frequency
combs.Comment: 11 pages, 7 figures. Accepted for publication in MNRA
Star formation and chemical evolution of damped Lyman systems
In this paper, we investigate the star formation and chemical evolution of
damped Lyman- systems (DLAs) based on the disc galaxy formation model
which is developed by Mo, Mao & White. We propose that the DLAs are the central
galaxies of less massive dark haloes present at redshifts , and they
should inhabit haloes of moderately low circular velocity. The empirical
Schmidt law of star formation rates, and closed box model of chemical evolution
that an approximation known as instantaneous recycling is assumed, are adopted.
In our models, when the predicted distribution of metallicity for DLAs is
calculated, two cases are considered. One is that, using the closed box model,
empirical Schmidt law and star formation time, the distribution of metallicity
can be directly calculated. The other is that, when the simple gravitational
instability of a thin isothermal gas disc as first discussed by Toomre is
considered, the star formation occurs only in the region where the surface
density of gas satisfies the critical value, not everywhere of a gas disc. We
assume that star formation in each DLA lasts for a period of 1 Gyr from
redshifts . There is only one output parameter in our models, i.e. the
stellar yield, which relates to the time of star formation history and is
obtained by normalizing the predicted distribution of metallicity to the mean
value of 1/13 as presented by Pettini et al. The predicted
metallicity distribution is consistent with the current (rather limited)
observational data. A random distribution of galactic discs is taken into
account.Comment: 7 pages, 6 figures, will appear in MNRAS, 2001 (21 April), 322,
927-93
Dissipationless Mergers of Elliptical Galaxies and the Evolution of the Fundamental Plane
We carry out numerical simulations of dissipationless major mergers of
elliptical galaxies using initial galaxy models that consist of a dark matter
halo and a stellar bulge with properties consistent with the observed
fundamental plane. By varying the density profile of the dark matter halo
(standard NFW versus adiabatically contracted NFW), the global stellar to dark
matter mass ratio, and the orbit of the merging galaxies, we are able to assess
the impact of each of these factors on the structure of the merger remnant. Our
results indicate that the properties of the remnant bulge depend primarily on
the angular momentum and energy of the orbit; for a cosmologically motivated
orbit, the effective radius and velocity dispersion of the remnant bulge remain
approximately on the fundamental plane. This indicates that the observed
properties of elliptical galaxies are consistent with significant growth via
late dissipationless mergers. We also find that the dark matter fraction within
the effective radius of our remnants increases after the merger, consistent
with the hypothesis that the tilt of the fundamental plane from the virial
theorem is due to a varying dark matter fraction as a function of galaxy mass.Comment: 14 pages, 6 figures; MNRAS, in press. Minor revisions, results from
an additional simulation adde
Mapping gene associations in human mitochondria using clinical disease phenotypes
Nuclear genes encode most mitochondrial proteins, and their mutations cause diverse and debilitating clinical disorders. To date, 1,200 of these mitochondrial genes have been recorded, while no standardized catalog exists of the associated clinical phenotypes. Such a catalog would be useful to develop methods to analyze human phenotypic data, to determine genotype-phenotype relations among many genes and diseases, and to support the clinical diagnosis of mitochondrial disorders. Here we establish a clinical phenotype catalog of 174 mitochondrial disease genes and study associations of diseases and genes. Phenotypic features such as clinical signs and symptoms were manually annotated from full-text medical articles and classified based on the hierarchical MeSH ontology. This classification of phenotypic features of each gene allowed for the comparison of diseases between different genes. In turn, we were then able to measure the phenotypic associations of disease genes for which we calculated a quantitative value that is based on their shared phenotypic features. The results showed that genes sharing more similar phenotypes have a stronger tendency for functional interactions, proving the usefulness of phenotype similarity values in disease gene network analysis. We then constructed a functional network of mitochondrial genes and discovered a higher connectivity for non-disease than for disease genes, and a tendency of disease genes to interact with each other. Utilizing these differences, we propose 168 candidate genes that resemble the characteristic interaction patterns of mitochondrial disease genes. Through their network associations, the candidates are further prioritized for the study of specific disorders such as optic neuropathies and Parkinson disease. Most mitochondrial disease phenotypes involve several clinical categories including neurologic, metabolic, and gastrointestinal disorders, which might indicate the effects of gene defects within the mitochondrial system. The accompanying knowledgebase (http://www.mitophenome.org/) supports the study of clinical diseases and associated genes
The non-perturbative regime of cosmic structure formation
This paper focusses on the barely understood gap between the weakly nonlinear
regime of structure formation and the onset of the virialized regime. While the
former is accessed through perturbative calculations and the latter through
virialization conditions incorporating dynamical stresses that arise in
collisionless self-gravitating systems due to velocity dispersion forces, the
addressed regime can only be understood through non-perturbative models. We
here present an exact Lagrangian integral that provides a tool to access this
regime. We derive a transport equation for the peculiar-gravitational field
strength and integrate it along comoving trajectories of fluid elements. The
so-obtained integral provides an exact expression that solves the longitudinal
gravitational field equation in general. We argue that this integral provides a
powerful approximation beyond the Lagrangian perturbative regime, and discuss
its relation to known approximations, among them Lagrangian perturbation
solutions including the Zel'dovich approximation and approximations for
adhesive gravitational clustering, including the adhesion approximation.
Furthermore, we propose an iteration scheme for a systematic analytical and
numerical construction of trajectory fields. The integral may also be employed
to improve inverse reconstruction techniques.Comment: 9 pages; matches published version in Astron. Astrophy
Antiepileptic drugs’ tolerability and safety – a systematic review and meta-analysis of adverse effects in dogs
<p>Various anti-epileptic drugs (AEDs) are used for the management of idiopathic epilepsy (IE) in dogs. Their safety profile is an important consideration for regulatory bodies, owners and prescribing clinicians. However, information on their adverse effects still remains limited with most of it derived from non-blinded non-randomized uncontrolled trials and case reports.</p><p><span>This poster won third place, which was presented at the Veterinary Evidence Today conference, Edinburgh November 1-3, 2016. </span></p><br /> <img src="https://www.veterinaryevidence.org/rcvskmod/icons/oa-icon.jpg" alt="Open Access" /
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