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Register a new userDiscovery of a shock front in the merging cluster of galaxies A2163
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ACO2163 is one of the hottest (mean $kT=12-15.5$ keV) and extremely X-ray overluminous merging galaxy clusters which is located at $z=0.203$. The cluster hosts one of the largest giant radio halos which are observed in most of the merging clusters, and a candidate radio relic. Recently, three merger shock fronts were detected in this cluster, explaining its extreme temperature and complex structure. Furthermore, previous XMM-Newton and Chandra observations hinted at the presence of a shock front that is associated with the gas `bullet crossing the main cluster in the west-ward direction, and which heated the intra-cluster medium, leading to adiabatic compression of the gas behind the bullet. The goal of this paper is to report on the detection of this shock front as revealed by the temperature discontinuity in the X-ray XMM-Newton image, and the edge in the Very Large Array (VLA) radio image. We also report on the detection of a relic source in the north-eastern region of the radio halo in the KAT-7 data, confirming the presence of an extended relic in this cluster. The brightness edge in the X-rays corresponds to a shock front with a Mach number $M= 2.2pm0.3$, at a distance of 0.2 Mpc from the cluster centre. An estimate from the luminosity jump gives $M=1.9pm0.4$. We consider a simple explanation for the electrons at the shock front, and for the observed discrepancy between the average spectral index of the radio halo emission and that predicted by the $M=2.2$ shock which precedes the bullet.
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Radio relics are Mpc-scale diffuse radio sources at the peripheries of galaxy clusters which are thought to trace outgoing merger shocks. We present XMM-Newton and Suzaku observations of the galaxy cluster Abell 2744 (z=0.306), which reveal the presence of a shock front 1.5 Mpc East of the cluster core. The surface-brightness jump coincides with the position of a known radio relic. Although the surface-brightness jump indicates a weak shock with a Mach number $mathcal{M}=1.7_{-0.3}^{+0.5}$, the plasma in the post-shock region has been heated to a very high temperature ($sim13$ keV) by the passage of the shock wave. The low acceleration efficiency expected from such a weak shock suggests that mildly relativistic electrons have been re-accelerated by the passage of the shock front.
We present the discovery of a single radio relic located at the edge of the galaxy cluster A2384, using the MeerKAT radio telescope. A2384 is a nearby ($z$ = 0.092), low mass, complex bimodal, merging galaxy cluster that displays a dense X-ray filament ($sim$ 700 kpc in length) between A2384(N) (Northern cluster) and A2384(S) (Southern cluster). The origin of the radio relic is puzzling. By using the MeerKAT observation of A2384, we estimate that the physical size of the radio relic is 824 $times$ 264 kpc$^{2}$ and that it is a steep spectrum source. The radio power of the relic is $P_{1.4mathrm{GHz}}$ $sim$ (3.87 $pm$ 0.40) $times$ 10$^{23}$ W Hz$^{-1}$. This radio relic could be the result of shock wave propagation during the passage of the low-mass A2384(S) cluster through the massive A2384(N) cluster, creating a trail appearing as a hot X-ray filament. In the previous GMRT 325 MHz observation we detected a peculiar FR I radio galaxy interacting with the hot X-ray filament of A2384, but the extended radio relic was not detected; it was confused with the southern lobe of the FR I galaxy. This newly detected radio relic is elongated and perpendicular to the merger axis, as seen in other relic clusters. In addition to the relic, we notice a candidate radio ridge in the hot X-ray filament. The physical size of the radio ridge source is $sim$ 182 $times$ 129 kpc$^{2}$. Detection of the diffuse radio sources in the X-ray filament is a rare phenomenon, and could be a new class of radio source found between the two merging clusters of A2384(N) and A2384(S).
The generation of turbulence at magnetized shocks and its subsequent interaction with the latter is a key question of plasma- and high-energy astrophysics. This paper presents two-dimensional magnetohydrodynamic simulations of a fast shock front interacting with incoming upstream perturbations, described as harmonic entropy or fast magnetosonic waves, both in the relativistic and the sub-relativistic regimes. We discuss how the disturbances are transmitted into downstream turbulence and we compare the observed response for small amplitude waves to a recent linear calculation. In particular, we demonstrate the existence of a resonant response of the corrugation amplitude when the group velocity of the outgoing downstream fast mode matches the velocity of the shock front. We also present simulations of large amplitude waves to probe the non-linear regime.
We present a new Chandra observation of the galaxy cluster Abell 2146 which has revealed a complex merging system with a gas structure that is remarkably similar to the Bullet cluster (eg. Markevitch et al. 2002). The X-ray image and temperature map show a cool 2-3 keV subcluster with a ram pressure stripped tail of gas just exiting the disrupted 6-7 keV primary cluster. From the sharp jump in the temperature and density of the gas, we determine that the subcluster is preceded by a bow shock with a Mach number M=2.2+/-0.8, corresponding to a velocity v=2200^{+1000}_{-900} km/s relative to the main cluster. We estimate that the subcluster passed through the primary core only 0.1-0.3 Gyr ago. In addition, we observe a slower upstream shock propagating through the outer region of the primary cluster and calculate a Mach number M=1.7+/-0.3. Based on the measured shock Mach numbers M~2 and the strength of the upstream shock, we argue that the mass ratio between the two merging clusters is between 3 and 4 to one. By comparing the Chandra observation with an archival HST observation, we find that a group of galaxies is located in front of the X-ray subcluster core but the brightest cluster galaxy is located immediately behind the X-ray peak.
We use the Bullet Cluster (1E0657-56) to investigate the extent to which star formation in cluster galaxies is influenced by ram pressure from supersonic gas (Mach 3) during a cluster merger. While the effects of ram pressure have been studied for individual galaxies infalling into galaxy clusters, this system provides a unique opportunity to investigate the impact of dramatic merger events on the cluster galaxy population. In this analysis we use {it Spitzer} IRAC data to study star formation. At the redshift of the cluster the 6.2 $mu$m PAH feature is redshifted into the 8 $mu$m band, enabling use of the m$_{4.5}$-m$_{8}$ color as a proxy for specific star formation rate. We find that the color distribution on the two sides of the shock differ by less than 2$sigma$, and conclude that ram pressure from the shock front has no dramatic, immediate impact on the star formation of cluster galaxies in the Bullet Cluster.
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