120 research outputs found
Suppression of thermal conduction in non-cooling flow clusters
Recent X-ray observations have revealed a universal temperature profile of
the intracluster gas of non-cooling flow clusters which is flat for r \le 0.2
r_{180}. Numerical simulations, however, obtain a steeper temperature profile
in the inner region. We study the effect of thermal conduction on the
intracluster gas in non-cooling flow clusters in light of these observations,
using the steep temperature profiles obtained by authors of numerical
simulations. We find that given 10^{10} yr for the intracluster gas to evolve,
thermal conduction should be suppressed from the classical value by a factor
\sim 10^{-3} in order to explain the observations.Comment: 5 pages, 3 figures, Accepted for publication in MNRAS (pink pages
Dynamical friction of radio galaxies in galaxy clusters
The distribution of luminous radio galaxies in galaxy clusters has been
observed to be concentrated in the inner region. We consider the role of
dynamical friction of massive galaxies ( M),
assuming them to be hosts of luminous radio galaxies, and show that beginning
with a Navarro-Frenk-White density profile of a cluster of mass M of concentration and collapsing at ,
the density profile of radio galaxies evolve to a profile of concentration , as observed, in a time scale of Gyr.Comment: 4 pages, 2 figures, MNRAS (Letters) in pres
Self-regulated black hole accretion, the M-sigma relation, and the growth of bulges in galaxies
We argue that the velocity dispersions and masses of galactic bulges and
spheroids are byproducts of the feedback that regulates rapid black hole growth
in protogalaxies. We suggest that the feedback energy liberated by accretion
must pass through the accreting material, in an energy-conserving flux close-in
and a momentum-conserving flux further out. If the inflowing gas dominates the
gravitational potential outside the Bondi radius, feedback from
Eddington-limited accretion drives the density profile of the gas to that of a
singular isothermal sphere. We find that the velocity dispersion associated
with the isothermal potential, sigma, increases with time as the black hole
mass M grows, in such a way that M is proportional to sigma^4. The coefficient
of this proportionality depends on the radius at which the flow switches from
energy conserving to momentum conserving, and gives the observed M-sigma
relation if the transition occurs at ~100 Schwarzschild radii. We associate
this transition with radiative cooling and show that bremsstrahlung, strongly
boosted by inverse Compton scattering in a two-temperature (T_p >> T_e) plasma,
leads to a transition at the desired radius.
According to this picture, bulge masses M_b are insensitive to the virial
masses of their dark matter haloes, but correlate linearly with black hole
mass. Our analytic model also explains the M_b-sigma (Faber-Jackson) relation
as a relic of black hole accretion. The model naturally explains why the
M-sigma relation has less scatter than either the M-M_b (Magorrian) or the
Faber-Jackson relation. It suggests that the M-sigma relation could extend down
to very low velocity dispersions, and predicts that the relation should not
evolve with redshift.Comment: 6 pages, no figures, submitted to Monthly Notices of the Royal
Astronomical Societ
Cooling flows and the entropy of the intragroup medium
We study steady, homogeneous and subsonic cooling flows in poor clusters of
galaxies in light of the recent proposal that radiative cooling of the
intracluster gas can explain the observations of the `entropy floor' and other
related X-ray observations. We study a family of cooling flow solutions
parameterized by the mass flux rate, and then determine the resulting entropy
and the X-ray luminosity. We find that cooling flows with mass flux rates in
excess of 1000 M_{solar} per yr are required to explain the observations of
entropy and X-ray luminosity. In view of the observed lack of such large flows
in rich clusters, our calculations suggest that heating sources are needed, in
addition to the effect of radiative cooling, to explain the observations.Comment: LaTeX with mn.sty, 7 figures, Accepted to MNRAS for publication (25th
October). Replaced by the final version with minor changes and typos
correcte
Narrow escape: how ionizing photons escape from disc galaxies
In this paper we calculate the escape fraction () of ionizing
photons from starburst galaxies. Using 2-D axisymmetric hydrodynamic
simulations, we study superbubbles created by overlapping supernovae in OB
associations. We calculate the escape fraction of ionizing photons from the
center of the disk along different angles through the superbubble and the gas
disk. After convolving with the luminosity function of OB associations, we show
that the ionizing photons escape within a cone of , consistent
with observations of nearby galaxies. The evolution of the escape fraction with
time shows that it falls initially as cold gas is accumulated in a dense shell.
After the shell crosses a few scale heights and fragments, the escape fraction
through the polar regions rises again. The angle-averaged escape fraction
cannot exceed from geometrical
considerations (using the emission cone opening angle). We calculate the
dependence of the time- and angle-averaged escape fraction on the mid-plane
disk gas density (in the range cm ) and the disk scale
height (between pc). We find that the escape fraction is related
to the disk parameters (the mid-plane disk density and scale height) roughly so
that (with ) is a constant.
For disks with a given WNM temperature, massive disks have lower escape
fraction than low mass galaxies. For Milky Way ISM parameters, we find , and it increases to for a galaxy ten times less
massive. We discuss the possible effects of clumpiness of the ISM on the
estimate of the escape fraction and the implications of our results for the
reionization of the universe.Comment: accepted for publication in MNRAS, 19 pages, 18 figure
Limits on the AGN activities in X-ray underluminous galaxy groups
We have observed four X-ray underluminous groups of galaxies using the Giant
Meterwave RadioTelescope. The groups NGC 524, 720, 3607, and 4697 are
underluminous in relation to the extrapolation of the Lx - T relation from rich
clusters and do not show any evidence of current AGN activities that can
account for such a departure. The GMRT observations carried out at low
frequencies (235 and 610 MHz) were aimed at detecting low surface brightness,
steep-spectrum sources indicative of past AGN activities in these groups. No
such radio emissions were detected in any of these four groups. The
corresponding upper limits on the total energy in relativistic particles is
about 3 X 10 erg. This value is more than a factor of 100 less than that
required to account for the decreased X-ray luminosities (or, enhanced
entropies) of these four groups in the AGN-heating scenario. Alternatively, the
AGN activity must have ceased about 4 Gyr ago, allowing the relativistic
particles to diffuse out to such a large extent (about 250 kpc) that their
radio emission could have been undetected by the current observations. If the
latter scenario is correct, the ICM was pre-heated before the assembly of
galaxy clusters.Comment: 10 pages, 3 figures, accepted for publication in ApJ Letter
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