65 research outputs found
Chandra Observation of the Merging Cluster A168: A Late Stage in the Evolution of a Cold Front
We present Chandra observations of the cool cluster A168, for which previous
X-ray imaging and optical studies indicated a merger of two subclusters nearly
in the plane of the sky. We derive a temperature map for A168, which shows that
the merger has proceeded beyond the core passage and is near subcluster
turnaround. It also reveals an unusual feature -- the gas core of one of the
subclusters forms a tongue-like structure extending ahead (in the direction of
motion) of the subcluster center. The coolest cluster gas is found in a
crescent-shaped region at the tip of this tongue, and forms a cold front in
pressure equilibrium with the external gas. In contrast with this feature's
forward location, previously observed merger cold fronts (e.g., A3667,
1E0657--56) lagged behind their host subclusters, as expected in the presense
of ram pressure. We propose that A168 illustrates a much later stage in the
evolution of a cold front, when its host subcluster approaches the apocenter of
the merger orbit where the ram pressure on its gas drops sharply. As a result,
a large chunk of the subcluster gas ``slingshots'' past the dark matter center,
becomes unbound from the subcluster and expands adiabatically, as seen in some
recent hydrodynamic simulations.Comment: 4 pages, 3 figures, to be published in ApJ Letter
Cluster Structure in Cosmological Simulations I: Correlation to Observables, Mass Estimates, and Evolution
We use Enzo, a hybrid Eulerian AMR/N-body code including non-gravitational
heating and cooling, to explore the morphology of the X-ray gas in clusters of
galaxies and its evolution in current generation cosmological simulations. We
employ and compare two observationally motivated structure measures: power
ratios and centroid shift. Overall, the structure of our simulated clusters
compares remarkably well to low-redshift observations, although some
differences remain that may point to incomplete gas physics. We find no
dependence on cluster structure in the mass-observable scaling relations, T_X-M
and Y_X-M, when using the true cluster masses. However, estimates of the total
mass based on the assumption of hydrostatic equilibrium, as assumed in
observational studies, are systematically low. We show that the hydrostatic
mass bias strongly correlates with cluster structure and, more weakly, with
cluster mass. When the hydrostatic masses are used, the mass-observable scaling
relations and gas mass fractions depend significantly on cluster morphology,
and the true relations are not recovered even if the most relaxed clusters are
used. We show that cluster structure, via the power ratios, can be used to
effectively correct the hydrostatic mass estimates and mass-scaling relations,
suggesting that we can calibrate for this systematic effect in cosmological
studies. Similar to observational studies, we find that cluster structure,
particularly centroid shift, evolves with redshift. This evolution is mild but
will lead to additional errors at high redshift. Projection along the line of
sight leads to significant uncertainty in the structure of individual clusters:
less than 50% of clusters which appear relaxed in projection based on our
structure measures are truly relaxed.Comment: 57 pages, 18 figures, accepted to ApJ, updated definition of T_X and
M_gas but results unchanged, for version with full resolution figures, see
http://www.ociw.edu/~tesla/sims.ps.g
Structure Shocks as a Source of Cosmic Rays in Clusters
Shocks are a ubiquitous consequence of cosmic structure formation, and they
play an essential role in heating galaxy cluster media. Virtually all of the
gas in clusters has been processed by one or more shocks of at least moderate
strength. These are collisionless shocks, so likely sites for diffusive shock
acceleration of high energy particles. We have carried out numerical
simulations of cosmic structure formation that directly include acceleration
and transport of nonthermal protons, as well as primary and secondary
electrons. Nonthermal emissions have also been computed from the resulting
particle spatial and energy distributions. Here we outline some of our current
findings, showing that nonthermal protons may contribute a significant pressure
in cluster media, and that expected radio, X-ray and -ray emissions
from these populations should be important cluster diagnostics.Comment: 14 pages with 5 figures. Invited talk presented at the "Matter and
Energy in Clusters of Galaxies" workshop in Taipei, 23-27 April, 2002. To
appear in the proceedings, published by PASP. eds: Chorng-Yuan Hwang and Stu
Bowye
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