221 research outputs found
Deconvolution of ASCA X-ray data: I. Spectral-imaging method
In this paper we describe a self-contained method for performing the
spectral-imaging deconvolution of X-ray data on clusters of galaxies observed
by the ASCA satellite. Spatially-resolved spectral studies of data from this
satellite require such a correction because its optics redistribute photons
over regions which are of comparable size to the angular scales of interest in
clusters. This scattering is a function not only of spatial position but also
energy. To perform a correction for these effects we employ Maximum-Likelihood
deconvolution of the image (within energy bands of 1 keV) to determine the
spatial redistribution, followed by a Monte-Carlo energy reassignment of photon
energies with position to determine the spectral redistribution. We present
tests on simulated cluster data, convolved with the various instrumental
characteristics and the X-ray background, which show that our methodology can
successfully recover a variety of intrinsic temperature profiles in typical
observational circumstances. In Paper-II we apply our spectral-imaging
deconvolution procedure to a large sample of galaxy clusters to determine
temperature profiles, some of which will be used in subsequent mass
determinations, presented in Paper-III.Comment: MNRAS, accepted. Paper and single page postscript copies of each
test's radial profile are available on: http://www-xray.ast.cam.ac.uk/~daw
Oxygen Absorption in Cooling Flows
The inhomogeneous cooling flow scenario predicts the existence of large
quantities of gas in massive elliptical galaxies, groups, and clusters that
have cooled and dropped out of the flow. Using spatially resolved, deprojected
X-ray spectra from the ROSAT PSPC we have detected strong absorption over
energies ~0.4-0.8 keV intrinsic to the central ~1 arcmin of the galaxy, NGC
1399, the group, NGC 5044, and the cluster, A1795. These systems have amongst
the largest nearby cooling flows in their respective classes and low Galactic
columns. Since no excess absorption is indicated for energies below ~0.4 keV
the most reasonable model for the absorber is warm, collisionally ionized gas
with T=10^{5-6} K where ionized states of oxygen provide most of the
absorption. Attributing the absorption only to ionized gas reconciles the large
columns of cold H and He inferred from Einstein and ASCA with the lack of such
columns inferred from ROSAT, and also is consistent with the negligible atomic
and molecular H inferred from HI, and CO observations of cooling flows. The
prediction of warm ionized gas as the product of mass drop-out in these and
other cooling flows can be verified by Chandra, XMM, and ASTRO-E.Comment: 4 pages (2 figures), Accepted for publication in ApJ Letters, no
significant changes from previous submitted versio
The Dark Matter Radial Profile in the Core of the Relaxed Cluster A2589
We present an analysis of a Chandra--ACIS observation of the galaxy cluster
A2589 to constrain the radial distribution of the total gravitating matter and
the dark matter in the core of the cluster. A2589 is especially well-suited for
this analysis because the hot gas in its core region (r < ~0.1 Rvir) is
undisturbed by interactions with a central radio source. From the largest
radius probed (r=0.07 Rvir) down to r ~0.02 Rvir dark matter dominates the
gravitating mass. Over this region the radial profiles of the gravitating and
dark matter are fitted well by the NFW and Hernquist profiles predicted by CDM.
The density profiles are also described well by power laws, rho ~r^{-alpha},
where alpha=1.37 +/- 0.14 for the gravitating matter and alpha=1.35 +/- 0.21
for the dark matter. These values are consistent with profiles of CDM halos but
are significantly larger than alpha ~0.5 found in LSB galaxies and expected
from self-interacting dark matter models.Comment: 10 pages, 6 figures, To Appear in The Astrophysical Journal, March 20
issue, a few very minor changes to match copyedited versio
The Evolution of Structure in X-ray Clusters of Galaxies
Using Chandra archival data, we quantify the evolution of cluster morphology
with redshift. Clusters form and grow through mergers with other clusters and
groups, and the amount of substructure in clusters in the present epoch and how
quickly it evolves with redshift depend on the underlying cosmology. Our sample
includes 40 X-ray selected, luminous clusters from the Chandra archive, and we
quantify cluster morphology using the power ratio method (Buote & Tsai 1995).
The power ratios are constructed from the moments of the X-ray surface
brightness and are related to a cluster's dynamical state. We find that, as
expected qualitatively from hierarchical models of structure formation,
high-redshift clusters have more substructure and are dynamically more active
than low-redshift clusters. Specifically, the clusters with z>0.5 have
significantly higher average third and fourth order power ratios than the lower
redshift clusters. Of the power ratios, is the most unambiguous
indicator of an asymmetric cluster structure, and the difference in
between the two samples remains significant even when the effects of noise and
other systematics are considered. After correcting for noise, we apply a linear
fit to versus redshift and find that the slope is greater than zero
at better than 99% confidence. This observation of structure evolution
indicates that dynamical state may be an important systematic effect in cluster
studies seeking to constrain cosmology, and when calibrated against numerical
simulations, structure evolution will itself provide interesting bounds on
cosmological models.Comment: 42 pages, 6 figures, ApJ accepted. For a version of the paper
containing an appendix with images of all of the clusters, see
http://www.ociw.edu/~tesla/structure.ps.g
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