326 research outputs found
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
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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