530 research outputs found
The Effects of Shear and Rotation Anisotropy Upon the Process of Gravitational Instability
In this paper, we study the role of shear fields on the evolution of density
perturbations embedded in a Friedmann flat background universe, by studying the
evolution of a homogeneous ellipsoid model. In this context, we show that while
the effect of the shear is that of increasing the growth rate of the density
contrast of a mass element, the angular momentum acquired by the ellipsoid has
the right magnitude to counterbalance the shear. Finally, our result show that
initial asphericities and tidal interaction induce a slowing down of the
collapse after the system has broken away from the general expansion, in
perfect agreement with the previrialization conjecture (Peebles & Groth 1976;
Davis & Peebles 1977)
Origin of gamma-ray emission in the shell of Cassiopeia A
Non-thermal X-ray emission from the shell of Cassiopeia A (Cas A) has been an
interesting subject of study, as it provides information about relativistic
electrons and their acceleration mechanisms in the shocks. Chandra X-ray
observatory revealed the detailed spectral and spatial structure of this SNR in
X-rays. The spectral analysis of Chandra X-ray data of Cas A shows unequal flux
levels for different regions of the shell, which can be attributed to different
magnetic fields in those regions. Additionally, the GeV gamma-ray emission
observed by Large Area Telescope on board Fermi Gamma Ray Space Telescope
showed that the hadronic processes are dominating in Cas A, a clear signature
of acceleration of protons. In this paper we aim to explain the GeV-TeV
gamma-ray data in the context of both leptonic and hadronic scenario. We
modeled the multi-wavelength spectrum of Cas A. We use synchrotron emission
process to explain the observed non-thermal X-ray fluxes from different regions
of the shell. These result in estimation of the model parameters, which are
then used to explain TeV gamma-ray emission spectrum. We also use hadronic
scenario to explain both GeV and TeV fluxes simultaneously. We show that a
leptonic model alone cannot explain the GeV-TeV data. Therefore, we need to
invoke a hadronic model to explain the observed GeV-TeV fluxes. We found that
although pure hadronic model is able to explain the GeV-TeV data, a
lepto-hadronic model provides the best fit to the data.Comment: Accepted in A&
Tracing circumnuclear dense gas in H2O maser galaxies
A sample of 30 H2O extra-galactic maser galaxies with their published
HCN(J=1-0) and HCO+(J=1-0) observations has been compiled to investigate the
dense gas correlation with H2O maser emission. Our sample number exceeds the
size of the previous HCN samples studied so far by a factor of three, and it is
the first study on the possible relation with the dense gas tracer HCO+. We
find a strong correlation between normalized H2O maser emission luminosity
(LH2O\LCO) and normalized HCO+ line luminosity (LHCO+\LCO). Moreover, a weak
correlation has been found between LH2O\LCO and normalized HCN line luminosity
(LHCN\LCO). The sample is also studied after excluding Luminous and
Ultraluminous infrared galaxy (U)LIRG sources, and the mentioned correlations
are noticeably stronger. We show that 'Dense gas' fractions as obtained from
HCN and HCO+ molecules tightly correlate with maser emission, especially for
galaxies with normal IR luminosity(LIR< 10^11Lsun) and we show that HCO+ is a
better 'dense gas' tracer than HCN. Further systematic studies of these dense
gas tracers with higher transition level lines are vital to probe megamaser
physical conditions and to accurately determining how maser emission
interrelates with the dense gas
Iron abundance distribution in the hot gas of merging galaxy clusters
We present XMM-Newton/EPIC observations of six merging galaxy clusters and
study the distributions of their temperature, iron (Fe) abundance and
pseudo-entropy along the merging axis. For the first time, we focus
simultaneously, and in a comprehensive way, on the chemical and thermodynamic
properties of the freshly collided intracluster medium (ICM). The Fe
distribution of these clusters along the merging axis is found to be in good
agreement with the azimuthally-averaged Fe abundance profile in typical
non-cool-core clusters out to . In addition to showing a moderate
central abundance peak, though less pronounced than in relaxed systems, the Fe
abundance flattens at large radii towards 0.2-0.3 . Although
this shallow metal distribution is in line with the idea that disturbed,
non-cool-core clusters originate from the merging of relaxed, cool-core
clusters, we find that in some cases, remnants of metal-rich and low entropy
cool cores can persist after major mergers. While we obtain a mild
anti-correlation between the Fe abundance and the pseudo-entropy in the (lower
entropy, = 200-500 keV cm) inner regions, no clear correlation is found
at (higher entropy, = 500-2300 keV cm) outer radii. The apparent
spatial abundance uniformity that we find at large radii is difficult to
explain through an efficient mixing of freshly injected metals, particularly in
systems for which the time since the merger is short. Instead, our results
provide important additional evidence in favour of the early enrichment
scenario - in which the bulk of the metals are released outside galaxies at
> 2-3 - and extend it from cool-core and (moderate) non-cool-core clusters to a
few of the most disturbed merging clusters as well. These results constitute a
first step towards a deeper understanding of the chemical history of merging
clusters.Comment: Accepted for publication in A&A, 21 pages with 17 figures and 19
table
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