373 research outputs found
Constraints on Cold H_2 Clouds from Gravitational Microlensing Searches
It has been proposed that the Galaxy might contain a population of cold
clouds in numbers sufficient to account for a substantial fraction of the total
mass of the Galaxy. These clouds would have masses of the order of 10^{-3}
Solar mass and sizes of the order of 10 AU. We consider here the lensing
effects of such clouds on the light from background stars. A semianalytical
formalism for calculation of the magnification event rate produced by such
gaseous lensing is developed, taking into account the spatial distribution of
the dark matter in the Galaxy, the velocity distribution of the lensing clouds
and source stars, and motion of the observer. Event rates are calculated for
the case of gaseous lensing of stars in the Large Magellanic Cloud and results
are directly compared with the results of the search for gravitational
microlensing events undertaken by the MACHO collaboration. The MACHO experiment
strongly constrains the properties of the proposed molecular clouds, but does
not completely rule them out. Future monitoring programs will either detect or
more strongly constrain this proposed population.Comment: 36 pages, 9 figures, 1 table, typos corrected, minor change
Gamma ray astronomy and baryonic dark matter
Recently, Dixon et al. have re-analyzed the EGRET data, finding a
statistically significant diffuse -ray emission from the galactic halo.
We show that this emission can naturally be explained within a
previously-proposed model for baryonic dark matter, in which -rays are
produced through the interaction of high-energy cosmic-ray protons with cold
clouds clumped into dark clusters - these dark clusters supposedly
populate the outer galactic halo and can show up in microlensing observations.
Our estimate for the halo -ray flux turns out to be in remarkably good
agreement with the discovery by Dixon et al. We also address future prospects
to test our predictions.Comment: 9 pages, 1 figure included, to appear in ApJ 510, L103 (1999
Hyperglycemia selectively increases the expression of cycloxygenase-2 in human aortic endothelial cells
The conversion of arachidonic acid to vasoactive prostanoids including prostacyclin, prostaglandins and tromboxanes is mediated by cycloxygenase (COX). Two isoforms of enzyme have been shown: a constitutive (COX-1) and an inducible form (COX-2). Products of the arachidonic acid metabolism may be involved in the impairment of endothelium-dependent vasodilatation observed both in experimental models and in patients with diabetes mellitus. To determine the effect of hyperglycemia on COX-1 and COX-2 expression, human aortic endothelial cells (HAEC) were exposed to normal (5.5mM) and high (22.2mM) concentrations of glucose for 5 days. Cells were also treated with mannitol (22.2 mM) to rule out an effect due to osmolality changes. COX-1 and COX-2 mRNA and protein expressions were analyzed by Southern and Western blotting, respectively. Treatment with high glucose was associated with a two-fold increase of both COX-2 mRNA (P<0.05) and protein levels (P<0.05), whereas no changes were observed for COX-1. Moreover high concentration of mannitol did not exert any significant effect. The present study demonstrates that both isoforms of COX are normally expressed in HAEC, but only COX-2 was stimulated after exposure to high glucose. The results of the present study may provide molecular basis to understand hyperglycemia-induced endothelial dysfunctio
Prevalence and Properties of Dark Matter in Elliptical Galaxies
Given the recently deduced relationship between X-ray temperatures and
stellar velocity dispersions (the "T-sigma relation") in an optically complete
sample of elliptical galaxies (Davis & White 1996), we demonstrate that L>L_*
ellipticals contain substantial amounts of dark matter in general. We present
constraints on the dark matter scale length and on the dark-to-luminous mass
ratio within the optical half-light radius and within the entire galaxy. For
example, we find that minimum values of dark matter core radii scale as r_dm >
4(L_V/3L_*)^{3/4}h^{-1}_80 kpc and that the minimum dark matter mass fraction
is >~20% within one optical effective radius r_e and is >~39-85% within 6r_e,
depending on the stellar density profile and observed value of beta_spec. We
also confirm the prediction of Davis & White (1996) that the dark matter is
characterized by velocity dispersions that are greater than those of the
luminous stars: sigma_dm^2 ~ 1.4-2 sigma_*^2. The T-sigma relation implies a
nearly constant mass-to-light ratio within six half-light radii: M/L_V ~ 25h_80
M_sun/L_V_sun. This conflicts with the simplest extension of CDM theories of
large scale structure formation to galactic scales; we consider a couple of
modifications which can better account for the observed T-sigma relation.Comment: 27 pages AASTeX; 15 PostScript figures; to appear in Ap
CMB as a possible new tool to study the dark baryons in galaxies
Baryons constitute about 4% of our universe, but most of them are missing and
we do not know where and in what form they are hidden. This constitute the
so-called missing baryon problem. A possibility is that part of these baryons
are hidden in galactic halos. We show how the 7-year data obtained by the WMAP
satellite may be used to trace the halo of the nearby giant spiral galaxy M31.
We detect a temperature asymmetry in the M31 halo along the rotation direction
up to about 120 kpc. This could be the first detection of a galactic halo in
microwaves and may open a new way to probe hidden baryons in these relatively
less studied galactic objects using high accuracy CMB measurements.Comment: 8 pages, presented at the III Italian-Pakistani Workshop on
Relativistic Astrophysics, Lecce, June 20-22, 2011; to be published in
Journal of Physics: Conference Serie
A Pressure Anomaly for HII Regions in Irregular Galaxies
The pressures of giant HII regions in 6 dwarf Irregular galaxies are a factor
of ~10 larger than the average pressures of the corresponding galaxy disks,
obtained from the stellar and gaseous column densities. Either the visible HII
regions in these dwarfs are all so young that they are still expanding, or
there is an unexpected source of disk self-gravity that increases the
background pressure. We consider the possibility that the additional
self-gravity comes from disk dark matter, but suggest this is unlikely because
the vertical scale heights inferred for Irregular galaxies are consistent with
the luminous matter alone. Some of the HII region overpressure is probably the
result of local peaks in the gravitational field that come from large gas
concentrations, many of which are observed directly. These peaks also explain
the anomalously low average column density thresholds for star formation that
were found earlier for Irregular galaxies, and they permit the existence of a
cool HI phase as the first step toward dense molecular cores. Many of the HII
regions could also be so strongly over-pressured that they will expand for a
long time. In this case, the observed population would be only 7% of the total,
and the aging HII regions, now too faint to see, should occupy nearly the
entire dwarf galaxy volume. Such prolonged HII region expansion would explain
the origin of the giant HI shells that are seen in these galaxies, and account
for the lack of bright central clusters inside these shells.Comment: 27 pages, 4 figures, Astrophysical Journal, 540, Sep 10, 2000, in
pres
Slott-Agape Project
SLOTT-AGAPE (Systematic Lensing Observation at Toppo Telescope - Andromeda
Gravitational Amplification Pixel Lensing Experiment) is a new collaboration
project among international partners from England, France, Germany, Italy and
Switzerland that intends to perform microlensing observation by using M31 as
target. The MACHOs search is made thanks to the pixel lensing technique.Comment: 4 pages, 2 figures, proceeding of XLIII Congresso della Societa'
Astronomica Italiana, Napoli, 4-8 Maggio, 199
Probing the mass function of halo dark matter via microlensing
The simplest interpretation of the microlensing events observed towards the
Large Magellanic Clouds is that approximately half of the mass of the Milky Way
halo is in the form of MAssive Compact Halo Objects with . It is not possible, due to limits from star counts and chemical
abundance arguments, for faint stars or white dwarves to comprise such a large
fraction of the halo mass. This leads to the consideration of more exotic lens
candidates, such as primordial black holes, or alternative lens locations. If
the lenses are located in the halo of the Milky Way, then constraining their
mass function will shed light on their nature. Using the current microlensing
data we find, for four halo models, the best fit parameters for delta-function,
primordial black hole and various power law mass functions. The best fit
primordial black hole mass functions, despite having significant finite width,
have likelihoods which are similar to, and for one particular halo model
greater than, those of the best fit delta functions . We then use Monte Carlo
simulations to investigate the number of microlensing events necessary to
determine whether the MACHO mass function has significant finite width. If the
correct halo model is known, then 500 microlensing events will be
sufficient, and will also allow determination of the mass function parameters
to .Comment: 28 pages including 14 figures, version to appear in ApJ, minor
changes to discussio
Sgr A: a laboratory to measure the central black hole and cluster parameters
Several stars orbit around a black hole candidate of mass
M, in the region of the Galactic Center (GC). Looking for General
Relativistic (GR) periastron shifts is limited by the existence of a stellar
cluster around the black hole that would modify the orbits due to classical
effects that might mask the GR effect. Only if one knows the cluster parameters
(its mass and core radius) it is possible to unequivocally deduce the GR
effects expected and then test them. In this paper it is shown that the
observation of the proper motion of Sgr A, km
s (\citealt{reid2004}), could help us to constrain the cluster
parameters significantly and that future measurements of the periastron shifts
for at least three stars may adequately determine the cluster parameters and
the mass of the black hole.Comment: in press on PASP, 200
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