620 research outputs found
Outskirts of Distant Galaxies In Absorption
QSO absorption spectroscopy provides a sensitive probe of both the neutral
medium and diffuse ionized gas in the distant Universe. It extends 21cm maps of
gaseous structures around low-redshift galaxies both to lower gas column
densities and to higher redshifts. Combining galaxy surveys with
absorption-line observations of gas around galaxies enables comprehensive
studies of baryon cycles in galaxy outskirts over cosmic time. This Chapter
presents a review of the empirical understanding of the cosmic neutral gas
reservoir from studies of damped Lya absorbers (DLAs). It describes the
constraints on the star formation relation and chemical enrichment history in
the outskirts of distant galaxies from DLA studies. A brief discussion of
available constraints on the ionized circumgalactic gas from studies of lower
column density Lya absorbers and associated ionic absorption transitions is
presented at the end.Comment: 45 pages, 7 figures, invited review, Book chapter in "Outskirts of
Galaxies", Eds. J. H. Knapen, J. C. Lee and A. Gil de Paz, Astrophysics and
Space Science Library, Springer, in pres
The role of endothelial cilia in post-embryonic vascular development
BACKGROUND: Cilia are essential for morphogenesis and maintenance of many tissues. Loss-of-function of cilia in early zebrafish development causes a range of vascular defects including cerebral haemorrhage and reduced arterial vascular mural cell coverage. In contrast, loss of endothelial cilia in mice has little effect on vascular development. We therefore used a conditional rescue approach to induce endothelial cilia ablation after early embryonic development and examined the effect on vascular development and mural cell development in post-embryonic, juvenile and adult zebrafish. RESULTS: ift54(elipsa) mutant zebrafish are unable to form cilia. We rescued cilia formation and ameliorated the phenotype of ift54 mutants using a novel Tg(ubi:loxP-ift54-loxP-myr-mcherry,myl7:EGFP)sh488 transgene expressing wildtype ift54 flanked by recombinase sites, then used a Tg(kdrl:cre)s898 transgene to induce endothelial-specific inactivation of ift54 at post-embryonic ages. Fish without endothelial ift54 function could survive to adulthood and exhibited no vascular defects. Endothelial inactivation of ift54 did not affect development of tagln-positive vascular mural cells around either the aorta or the caudal fin vessels, nor formation of vessels after tailfin resection in adult animals. CONCLUSIONS: Endothelial cilia are not essential for development and remodelling of the vasculature in juvenile and adult zebrafish when inactivated after embryogenesis
Prospects For Identifying Dark Matter With CoGeNT
It has previously been shown that the excess of events reported by the CoGeNT
collaboration could be generated by elastically scattering dark matter
particles with a mass of approximately 5-15 GeV. This mass range is very
similar to that required to generate the annual modulation observed by
DAMA/LIBRA and the gamma rays from the region surrounding the Galactic Center
identified within the data of the Fermi Gamma Ray Space Telescope. To
confidently conclude that CoGeNT's excess is the result of dark matter,
however, further data will likely be needed. In this paper, we make projections
for the first full year of CoGeNT data, and for its planned upgrade. Not only
will this body of data more accurately constrain the spectrum of nuclear recoil
events, and corresponding dark matter parameter space, but will also make it
possible to identify seasonal variations in the rate. In particular, if the
CoGeNT excess is the product of dark matter, then one year of CoGeNT data will
likely reveal an annual modulation with a significance of 2-3. The
planned CoGeNT upgrade will not only detect such an annual modulation with high
significance, but will be capable of measuring the energy spectrum of the
modulation amplitude. These measurements will be essential to irrefutably
confirming a dark matter origin of these events.Comment: 6 pages, 6 figure
Dependence of direct detection signals on the WIMP velocity distribution
The signals expected in WIMP direct detection experiments depend on the
ultra-local dark matter distribution. Observations probe the local density,
circular speed and escape speed, while simulations find velocity distributions
that deviate significantly from the standard Maxwellian distribution. We
calculate the energy, time and direction dependence of the event rate for a
range of velocity distributions motivated by recent observations and
simulations, and also investigate the uncertainty in the determination of WIMP
parameters. The dominant uncertainties are the systematic error in the local
circular speed and whether or not the MW has a high density dark disc. In both
cases there are substantial changes in the mean differential event rate and the
annual modulation signal, and hence exclusion limits and determinations of the
WIMP mass. The uncertainty in the shape of the halo velocity distribution is
less important, however it leads to a 5% systematic error in the WIMP mass. The
detailed direction dependence of the event rate is sensitive to the velocity
distribution. However the numbers of events required to detect anisotropy and
confirm the median recoil direction do not change substantially.Comment: 21 pages, 7 figures, v2 version to appear in JCAP, minor change
Spin-independent elastic WIMP scattering and the DAMA annual modulation signal
We discuss the interpretation of the annual modulation signal seen in the
DAMA experiment in terms of spin-independent elastic WIMP scattering. Taking
into account channeling in the crystal as well as the spectral signature of the
modulation signal we find that the low-mass WIMP region consistent with DAMA
data is confined to WIMP masses close to GeV, in
disagreement with the constraints from CDMS and XENON. We conclude that even if
channeling is taken into account this interpretation of the DAMA modulation
signal is disfavoured. There are no overlap regions in the parameter space at
90% CL and a consistency test gives the probability of . We
study the robustness of this result with respect to variations of the WIMP
velocity distribution in our galaxy, by changing various parameters of the
distribution function, and by using the results of a realistic N-body dark
matter simulation. We find that only by making rather extreme assumptions
regarding halo properties can we obtain agreement between DAMA and CDMS/XENON.Comment: 21 pages, 12 figures, matches version accepted in JCA
An Atom Laser Based on Raman Transitions
In this paper we present an atom laser scheme using a Raman transition for
the output coupling of atoms. A beam of thermal atoms (bosons) in a metastable
atomic state are pumped into a multimode atomic cavity. This cavity is
coupled through spontaneous emission to a single mode of another cavity for the
ground atomic state, . Above a certain threshold pumping rate a large
number of atoms, , builds up in this single quantum state and transitions
to the ground state of the cavity become enhanced by a factor .
Atoms in this state are then coupled to the outside of the cavity with a Raman
transition. This changes the internal state of the atom and imparts a momentum
kick, allowing the atoms to leave the system.Comment: 8 pages, 4 postscript figures, uses RevTex, home page at
http://online.anu.edu.au/Physics/Welcome.html (Some aspects of the exact
physical model have changed from original version. Other general improvements
included
Diffusion as mixing mechanism in granular materials
We present several numerical results on granular mixtures. In particular, we
examine the efficiency of diffusion as a mixing mechanism in these systems. The
collisions are inelastic and to compensate the energy loss, we thermalize the
grains by adding a random force. Starting with a segregated system, we show
that uniform agitation (heating) leads to a uniform mixture of grains of
different sizes. We define a characteristic mixing time, , and
study theoretically and numerically its dependence on other parameters like the
density. We examine a model for bidisperse systems for which we can calculate
some physical quantities. We also examine the effect of a temperature gradient
and demonstrate the appearance of an expected segregation.Comment: 15 eps figures, include
A Bayesian view of the current status of dark matter direct searches
Bayesian statistical methods offer a simple and consistent framework for
incorporating uncertainties into a multi-parameter inference problem. In this
work we apply these methods to a selection of current direct dark matter
searches. We consider the simplest scenario of spin-independent elastic WIMP
scattering, and infer the WIMP mass and cross-section from the experimental
data with the essential systematic uncertainties folded into the analysis. We
find that when uncertainties in the scintillation efficiency of Xenon100 have
been accounted for, the resulting exclusion limit is not sufficiently
constraining to rule out the CoGeNT preferred parameter region, contrary to
previous claims. In the same vein, we also investigate the impact of
astrophysical uncertainties on the preferred WIMP parameters. We find that
within the class of smooth and isotropic WIMP velocity distributions, it is
difficult to reconcile the DAMA and the CoGeNT preferred regions by tweaking
the astrophysics parameters alone. If we demand compatibility between these
experiments, then the inference process naturally concludes that a high value
for the sodium quenching factor for DAMA is preferred.Comment: 37 pages, 14 figures and 7 tables. Replacement for matching the
version accepted for publicatio
Cerebrovascular endothelial cells form transient Notchâdependent cystic structures in zebrafish
We identify a novel endothelial membrane behaviour in transgenic zebrafish. Cerebral blood vessels extrude large transient spherical structures that persist for an average of 23 min before regressing into the parent vessel. We term these structures âkugelnâ, after the German for sphere. Kugeln are only observed arising from the cerebral vessels and are present as late as 28 days post fertilization. Kugeln do not communicate with the vessel lumen and can form in the absence of blood flow. They contain little or no cytoplasm, but the majority are highly positive for nitric oxide reactivity. Kugeln do not interact with brain lymphatic endothelial cells (BLECs) and can form in their absence, nor do they perform a scavenging role or interact with macrophages. Inhibition of actin polymerization, Myosin II, or Notch signalling reduces kugel formation, while inhibition of VEGF or Wnt dysregulation (either inhibition or activation) increases kugel formation. Kugeln represent a novel Notchâdependent NOâcontaining endothelial organelle restricted to the cerebral vessels, of currently unknown function
Ice sheet model dependency of the simulated Greenland Ice Sheet in the mid-Pliocene
The understanding of the nature and behavior of ice sheets in past warm periods is important for constraining the potential impacts of future climate change. The Pliocene warm period (between 3.264 and 3.025 Ma) saw global temperatures similar to those projected for future climates; nevertheless, Pliocene ice locations and extents are still poorly constrained. We present results from the efforts to simulate mid-Pliocene Greenland Ice Sheets by means of the international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP). We compare the performance of existing numerical ice sheet models in simulating modern control and mid-Pliocene ice sheets with a suite of sensitivity experiments guided by available proxy records. We quantify equilibrated ice sheet volume on Greenland, identifying a potential range in sea level contributions from warm Pliocene scenarios. A series of statistical measures are performed to quantify the confidence of simulations with focus on inter-model and inter-scenario differences. We find that Pliocene Greenland Ice Sheets are less sensitive to differences in ice sheet model configurations and internal physical quantities than to changes in imposed climate forcing. We conclude that Pliocene ice was most likely to be limited to the highest elevations in eastern and southern Greenland as simulated with the highest confidence and by synthesizing available regional proxies; however, the extent of those ice caps needs to be further constrained by using a range of general circulation model (GCM) climate forcings
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