120 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
Cosmological Blastwaves and the Intergalactic Medium
Winds from protogalactic starbursts and quasars can drive shocks that heat,
ionize, and enrich the intergalactic medium. The Sedov-Taylor solution for
point-like explosions adequately describes these blastwaves early in their
development, but as the time since the explosion () approaches the age
of the universe (), cosmological effects begin to alter the blastwave's
structure and growth rate. This paper presents an analytical solution for
adiabatic blastwaves in an expanding universe, valid when the IGM is
homogeneous and contains only a small fraction of the total mass density
(). Using this analytical solution, we examine
the role protogalactic explosions might play in determining the state of
intergalactic gas at .Comment: 37 pages, 11 Postscript figures, LaTeX aaspp.sty file; to appear in
Astrophysical Journal, in press (1996 July 10
On Nulling Interferometers and the Line-Emitting Regions of AGNs
The nulling interferometers proposed to study planets around other stars are
generally well suited for studying small-scale structures surrounding other
bright pointlike objects such as the nuclei of active galaxies. Conventional
interferometric techniques will produce useful maps of optical/IR line and
continuum emission within active galaxies on scales of 10 milliarcseconds
(mas), but similar studies of broad-line regions will require baselines longer
than those currently envisaged. Nevertheless, nulling interferometers currently
under development will be able to constrain quasar velocity fields on
milliarcsecond scales, as long as they are equipped with spectrographs capable
of resolving lines several hundred km/s wide. This Letter describes how
analyses of line emission leaking through the edges of the null in such an
instrument can reveal the size, shape, and velocity field of nebular gas on the
outskirts of a quasar broad-line region. If this technique proves effective, it
could potentially be used to measure the mass function of quasar black holes
throughout the universe.Comment: Latex, 9 pages, 2 figures, to appear in 1 October ApJ Letter
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
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