618 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
Runaway Merger Shocks in Galaxy Cluster Outskirts and Radio Relics
Moderately strong shocks arise naturally when two subclusters merge. For
instance, when a smaller subcluster falls into the gravitational potential of a
more massive cluster, a bow shock is formed and moves together with the
subcluster. After pericenter passage, however, the subcluster is decelerated by
the gravity of the main cluster, while the shock continues moving away from the
cluster center. These shocks are considered as promising candidates for
powering radio relics found in many clusters. The aim of this paper is to
explore the fate of such shocks when they travel to the cluster outskirts, far
from the place where the shocks were initiated. In a uniform medium, such a
"runaway" shock should weaken with distance. However, as shocks move to large
radii in galaxy clusters, the shock is moving down a steep density gradient
that helps the shock to maintain its strength over a large distance.
Observations and numerical simulations show that, beyond , gas density
profiles are as steep as, or steeper than, , suggesting that there
exists a "Habitable zone" for moderately strong shocks in cluster outskirts
where the shock strength can be maintained or even amplified. A characteristic
feature of runaway shocks is that the strong compression, relative to the
initial state, is confined to a narrow region just behind the shock. Therefore,
if such a shock runs over a region with a pre-existing population of
relativistic particles, then the boost in radio emissivity, due to pure
adiabatic compression, will also be confined to a narrow radial shell.Comment: 9 pages, 8 figures; published in MNRA
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
Role of Electon Excitation and Nature of Molecular Gas in Cluster Central Elliptical Galaxies
We present observations in CO(3-2) that, combined with previous observations
in CO(2-1), constrain the physical properties of the filamentary molecular gas
in the central 6.5 kpc of NGC 1275, the central giant elliptical galaxy
of the Perseus cluster. We find this molecular gas to have a temperature
K and a density -, typically
warmer and denser than the bulk of Giant Molecular Clouds (GMCs) in the Galaxy.
Bathed in the harsh radiation and particle field of the surrounding
intracluster X-ray gas, the molecular gas likely has a much higher ionization
fraction than that of GMCs. For an ionization fraction of ,
similar to that of Galactic diffuse ()
partially-molecular clouds that emit in HCN(1-0) and HCO(1-0), we show that
the same gas traced in CO can produce the previously reported emissions in
HCN(3-2), HCO(3-2), and CN(2-1) from NGC 1275; the dominant source of
excitation for all the latter molecules is collisions with electrons. To
prevent collapse, as evidenced by the lack of star formation in the molecular
filaments, they must consist of thin strands that have cross-sectional radii
0.2-2 pc if supported solely by thermal gas pressure; larger radii
are permissible if turbulence or poloidal magnetic fields provide additional
pressure support. We point out that the conditions required to relate CO
luminosities to molecular gas masses in our Galaxy are unlikely to apply in
cluster central elliptical galaxies. Rather than being virialized structures
analogous to GMCs, we propose that the molecular gas in NGC 1275 comprises
pressure-confined structures created by turbulent flows.Comment: 41 pages, 1 table, 12 figures; accepted by Ap
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
Elemental Abundances in the Intracluster Gas and the Hot Galactic Coronae in Cluster A194
We have completed the analysis of observations of the Coma cluster and are continuing analysis of A1367 both of which are shown to be merging clusters. Also, we are analyzing observations of the Centaurus cluster which we see as a merger based in both its temperature and surface brightness distributions. Attachment: Another collision for the coma cluster
Are Large Core Radius Clusters Merging Systems?
We have analyzed observations for two lensing clusters of galaxies, A1689 and A2218. Our investigations have explored the implications of their X-ray properties for mass determinations both in X-rays and through both weak and strong gravitational lensing. The work on these two clusters is summarized below and copies of the two papers submitted to the Astrophysical Journal and accepted for publication are attached
Deep Chandra observations of NGC 1404 : cluster plasma physics revealed by an infalling early-type galaxy
The intracluster medium (ICM), as a magnetized and highly ionized fluid, provides an ideal laboratory to study plasma physics under extreme conditions that cannot yet be achieved on Earth. NGC 1404 is a bright elliptical galaxy that is being gas stripped as it falls through the ICM of the Fornax Cluster. We use the new {\sl Chandra} X-ray observations of NGC 1404 to study ICM microphysics. The interstellar medium (ISM) of NGC 1404 is characterized by a sharp leading edge, 8 kpc from the galaxy center, and a short downstream gaseous tail. Contact discontinuities are resolved on unprecedented spatial scales (0\farcs5=45\,pc) due to the combination of the proximity of NGC 1404, the superb spatial resolution of {\sl Chandra}, and the very deep (670 ksec) exposure. At the leading edge, we observe sub-kpc scale eddies generated by Kelvin-Helmholtz instability and put an upper limit of 5\% Spitzer on the isotropic viscosity of the hot cluster plasma. We also observe mixing between the hot cluster gas and the cooler galaxy gas in the downstream stripped tail, which provides further evidence of a low viscosity plasma. The assumed ordered magnetic fields in the ICM ought to be smaller than 5\,μG to allow KHI to develop. The lack of evident magnetic draping layer just outside the contact edge is consistent with such an upper limit
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