186 research outputs found
Substructure: Clues to the Formation of Clusters of Galaxies
We have examined the spatial distribution of substructure in clusters of
galaxies using Einstein X-ray observations. Subclusters are found to have a
markedly anisotropic distribution that reflects the surrounding matter
distribution on supercluster scales. Our results suggest a picture in which
cluster formation proceeds by mergers of subclusters along large-scale
filaments. The implications of such an anisotropic formation process for the
shapes, orientations and kinematics of clusters are discussed briefly.Comment: 7 pages, uuencoded compressed postscript. To appear in ApJ Letters
(September 20, 1995 issue
Standoff Distance of Bow Shocks in Galaxy Clusters as Proxy for Mach Number
X-ray observations of merging clusters provide many examples of bow shocks
leading merging subclusters. While the Mach number of a shock can be estimated
from the observed density jump using Rankine-Hugoniot condition, it reflects
only the velocity of the shock itself and is generally not equal to the
velocity of the infalling subcluster dark matter halo or to the velocity of the
contact discontinuity separating gaseous atmospheres of the two subclusters.
Here we systematically analyze additional information that can be obtained by
measuring the standoff distance, i.e. the distance between the leading edge of
the shock and the contact discontinuity that drives this shock. The standoff
distance is influenced by a number of additional effects, e.g. (1) the
gravitational pull of the main cluster (causing acceleration/deceleration of
the infalling subcluster), (2) the density and pressure gradients of the
atmosphere in the main cluster, (3) the non-spherical shape of the subcluster,
and (4) projection effects. The first two effects tend to bias the standoff
distance in the same direction, pushing the bow shock closer to (farther away
from) the subcluster during the pre- (post-)merger stages. Particularly, in the
post-merger stage, the shock could be much farther away from the subcluster
than predicted by a model of a body moving at a constant speed in a uniform
medium. This implies that a combination of the standoff distance with
measurements of the Mach number from density/temperature jumps can provide
important information on the merger, e.g. differentiating between the pre- and
post-merger stages.Comment: 11 pages, 12 figures. Including major revision and matched to
accepted version in MNRA
Intracluster Globular Clusters
Globular cluster populations of supergiant elliptical galaxies are known to
vary widely, from extremely populous systems like that of UGC 9799, the
centrally dominant galaxy in Abell 2052, to globular-cluster-poor galaxies such
as NGC 5629 in Abell 2666. Here we propose that these variations point strongly
to the existence of a population of globular clusters that are not bound to
individual galaxies, but rather move freely throughout the cores of clusters of
galaxies. Such intracluster globular clusters may have originated as tidally
stripped debris from galaxy interactions and mergers, or alternatively they may
have formed in situ in some scenarios of globular cluster formation.Comment: 9 pages, uuencoded compressed postscript. Accepted for publication in
the Astrophysical Journal Letter
X-ray scaling relations from a complete sample of the richest maxBCG clusters
We use a complete sample of 38 richest maxBCG clusters to study the
ICM-galaxy scaling relations and the halo mass selection properties of the
maxBCG algorithm, based on X-ray and optical observations. The clusters are
selected from the two largest bins of optical richness in the Planck stacking
work with the maxBCG richness . We analyze their Chandra and
XMM-Newton data to derive the X-ray properties of the ICM. We then use the
distribution of , , to study the mass selection
of maxBCG. Compared with previous works based on the whole richness
sample, a significant fraction of blended systems with boosted richness is
skewed into this richest sample. Parts of the blended halos are picked apart by
the redMaPPer, an updated red-sequence cluster finding algorithm with lower
mass scatter. Moreover, all the optical blended halos are resolved as
individual X-ray halos, following the established and
relations. We further discuss that the discrepancy between ICM-galaxy scaling
relations, especially for future blind stacking, can come from several factors,
including miscentering, projection, contamination of low mass systems, mass
bias and covariance bias. We also evaluate the fractions of relaxed and cool
core clusters in our sample. Both are smaller than those from SZ or X-ray
selected samples. Moreover, disturbed clusters show a higher level of mass bias
than relaxed clusters.Comment: 28 pages, 12 figures, MNRAS in pres
Detection of a Star Forming Galaxy in the Center of a Low-Mass Galaxy Cluster
Brightest Cluster Galaxies (BCGs) residing in the centers of galaxy clusters
are typically quenched giant ellipticals. A recent study hinted that
star-forming galaxies with large disks, so-called superluminous spirals and
lenticulars, are the BCGs of a subset of galaxy clusters. Based on the existing
optical data it was not possible to constrain whether the superluminous disk
galaxies reside at the center of galaxy clusters. In this work, we utilize
XMM-Newton X-ray observations of five galaxy clusters to map the morphology of
the intracluster medium (ICM), characterize the galaxy clusters, determine the
position of the cluster center, and measure the offset between the cluster
center and the superluminous disk galaxies. We demonstrate that one
superluminous lenticular galaxy, 2MASX J10405643-0103584, resides at the center
of a low-mass () galaxy cluster. This
represents the first conclusive evidence that a superluminous disk galaxy is
the central BCG of a galaxy cluster. We speculate that the progenitor of 2MASX
J10405643-0103584 was an elliptical galaxy, whose extended disk was re-formed
due to the merger of galaxies. We exclude the possibility that the other four
superluminous disk galaxies reside at the center of galaxy clusters, as their
projected distance from the cluster center is kpc, which corresponds
to . We conclude that these clusters host quiescent
massive elliptical galaxies at their center.Comment: 7 pages, 3 figures, accepted for publication in the Astrophysical
Journa
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