2,876 research outputs found
Cooling and Clusters: When Is Heating Needed?
There are (at least) two unsolved problems concerning the current state of
the thermal gas in clusters of galaxies. The first is identifying the source of
the heating which offsets cooling in the centers of clusters with short cooling
times (the ``cooling flow'' problem). The second is understanding the mechanism
which boosts the entropy in cluster and group gas. Since both of these problems
involve an unknown source of heating it is tempting to identify them with the
same process, particular since AGN heating is observed to be operating at some
level in a sample of well-observed ``cooling flow'' clusters. Here we show,
using numerical simulations of cluster formation, that much of the gas ending
up in clusters cools at high redshift and so the heating is also needed at
high-redshift, well before the cluster forms. This indicates that the same
process operating to solve the cooling flow problem may not also resolve the
cluster entropy problem.Comment: 10 pages, 5 figures, published in Philosophical Transactions A (Royal
Society
Confusion of Diffuse Objects in the X-ray Sky
Most of the baryons in the present-day universe are thought to reside in
intergalactic space at temperatures of 10^5-10^7 K. X-ray emission from these
baryons contributes a modest (~10%) fraction of the ~ 1 keV background whose
prominence within the large-scale cosmic web depends on the amount of
non-gravitational energy injected into intergalactic space by supernovae and
AGNs. Here we show that the virialized regions of groups and clusters cover
over a third of the sky, creating a source-confusion problem that may hinder
X-ray searches for individual intercluster filaments and contaminate
observations of distant groups.Comment: accepted to ApJ Letters, 7 pages, 3 figure
The baseline intracluster entropy profile from gravitational structure formation
The radial entropy profile of the hot gas in clusters of galaxies tends to
follow a power law in radius outside of the cluster core. Here we present a
simple formula giving both the normalization and slope for the power-law
entropy profiles of clusters that form in the absence of non-gravitational
processes such as radiative cooling and subsequent feedback. It is based on
seventy-one clusters drawn from four separate cosmological simulations, two
using smoothed-particle hydrodynamics (SPH) and two using adaptive-mesh
refinement (AMR), and can be used as a baseline for assessing the impact of
non-gravitational processes on the intracluster medium outside of cluster
cores. All the simulations produce clusters with self-similar structure in
which the normalization of the entropy profile scales linearly with cluster
temperature, and these profiles are in excellent agreement outside of 0.2
r_200. Because the observed entropy profiles of clusters do not scale linearly
with temperature, our models confirm that non-gravitational processes are
necessary to break the self-similarity seen in the simulations. However, the
core entropy levels found by the two codes used here significantly differ, with
the AMR code producing nearly twice as much entropy at the centre of a cluster.Comment: Accepted to MNRAS, 8 pages, 9 figure
Deployment-related mild traumatic brain injury, mental health problems, and post-concussive symptoms in Canadian armed forces personnel
The X-ray surface brightness distribution from diffuse gas
We use simulations to predict the X-ray surface brightness distribution
arising from hot, cosmologically distributed diffuse gas. The distribution is
computed for two bands: 0.5-2 keV and 0.1-0.4 keV, using a
cosmological-constant dominated cosmology that fits many other observations. We
examine a number of numerical issues such as resolution, simulation volume and
pixel size and show that the predicted mean background is sensitive to
resolution such that higher resolution systematically increases the mean
predicted background. Although this means that we can compute only lower bounds
to the predicted level, these bounds are already quite restrictive. Since the
observed extra-galactic X-ray background is mostly accounted for by compact
sources, the amount of the observed background attributable to diffuse gas is
tightly constrained. We show that without physical processes in addition to
those included in the simulations (such as radiative cooling or
non-gravitational heating), both bands exceed observational limits. In order to
examine the effect of non-gravitational heating we explore a simple modeling of
energy injection and show that substantial amounts of heating are required
(i.e. 5 keV per particle when averaged over all baryons). Finally, we also
compute the distribution of surface brightness on the sky and show that it has
a well-resolved characteristic shape. This shape is substantially modified by
non-gravitational heating and can be used as a probe of such energy injection.Comment: 11 pages, 11 figures, submitted to Ap
On the Distribution of X-ray Surface Brightness from Diffuse Gas
Hot intergalactic gas in clusters, groups, and filaments emanates a
continuous background of 0.5-2.0 keV X-rays that ought to be detectable with
the new generation of X-ray observatories. Here we present selected results
from a program to simulate the surface-brightness distribution of this
background with an adaptive-mesh cosmological hydrodynamics code. We show that
the bright end of this distribution is well approximated by combining the
cluster temperature function with a beta-model for surface brightness and
appropriate luminosity-temperature and core radius-luminosity relations. Our
simulations verify that the X-ray background from hot gas vastly exceeds
observational limits if non-gravitational processes do not modify the
intergalactic entropy distribution. An entropy floor of ~100 keV cm^2, which
could be established by either heating or cooling, appears necessary to
reconcile the simulated background with observations. Because the X-ray
background distribution is so sensitive to the effects of non-gravitational
processes, it offers a way to constrain the thermal history of the
intergalactic medium that is independent of the uncertainties associated with
surveys of clusters and groups.Comment: 7 pages, 3 figures, submitted to ApJ Letters Nov 17, 200
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