393 research outputs found
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
Microlensing Halo Models with Abundant Brown Dwarfs
All previous attempts to understand the microlensing results towards the
Large Magellanic Cloud (LMC) have assumed homogeneous present day mass
functions (PDMFs) for the lensing populations. Here, we present an
investigation into the microlensing characteristics of haloes with spatially
varying PDMFs and anisotropic velocity dispersion tensors. One attractive
possibility -- suggested by baryonic dark cluster formation in pregalactic and
protogalactic cooling flows -- is that the inner halo is dominated by stellar
mass objects, whereas low mass brown dwarfs become more prevalent on moving
outwards. The contribution to the microlensing rate must be dominated by dark
remnants (of about 0.5 solar masses) to recover the observed timescales of the
microlensing experiments. But, even though stellar remnants control the rate,
they do not dominate the mass of the baryonic halo, and so the well-known
enrichment and mass budget problems are much less severe. Using a simple ansatz
for the spatial variation of the PDMF, models are constructed in which the
contribution of brown dwarfs to the mass of the baryonic halo is 55 % and to
the total halo is 30 %. An unusual property of the models is that they predict
that the average timescale of events towards M31 is shorter than the average
timescale towards the LMC. This is because the longer line of sight towards M31
probes more of the far halo where brown dwarfs are the most common constituent.Comment: 17 pages, 1 figure, in press at The Astrophysical Journal (Letters
Introduzione
L'introduzione espone le ragioni scientifiche della pubblicazione del volume contenente gli Atti del IV seminario per giovani studiosi di letteratura latina con contributi su Livio, Cesare, Apuleio, Lucrezio, Varrone, Plauto, Ps. Acrone, Servio, Lucano, Manilio
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
Planck's confirmation of the M31 disk and halo rotation
Planck's data acquired during the first 15.4 months of observations towards
both the disk and halo of the M31 galaxy are analyzed. We confirm the existence
of a temperature asymmetry, previously detected by using the 7-year WMAP data,
along the direction of the M31 rotation, therefore indicative of a
Doppler-induced effect. The asymmetry extends up to about 10 degrees (about 130
kpc) from the M31 center. We also investigate the recent issue raised in Rubin
and Loeb (2014) about the kinetic Sunyaev-Zeldovich effect from the diffuse hot
gas in the Local Group, predicted to generate a hot spot of a few degrees size
in the CMB maps in the direction of M31, where the free electron optical depth
gets the maximum value. We also consider the issue whether in the opposite
direction with respect to the M31 galaxy the same effect induces a minimum in
temperature in the Planck's maps of the sky. We find that the Planck's data at
100 GHz show an effect even larger than that expected.Comment: 4 pages, 1 table, 2 figures, in press as a Letter in A&
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
Strong field limit analysis of gravitational retro-lensing
We present a complete treatment in the strong field limit of gravitational
retro-lensing by a static spherically symmetric compact object having a photon
sphere. The results are compared with those corresponding to ordinary lensing
in similar strong field situations. As examples of application of the
formalism, a supermassive black hole at the galactic center and a stellar mass
black hole in the galactic halo are studied as retro-lenses, in both cases
using the Schwarzschild and Reissner-Nordstrom geometries.Comment: 11 pages, 1 figure; v2: minor changes. Accepted for publication in
Physical Review
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
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