A Merger Shock in Abell 2034

We present a $250,$ks Chandra observation of the cluster merger A2034 with the aim of understanding the nature of a sharp edge previously characterized as a cold front. The new data reveal that the edge is coherent over a larger opening angle and is significantly more bow-shock-shaped than previously thought. Within $sim 27,$degrees about the axis of symmetry of the edge the density, temperature and pressure drop abruptly by factors of $1.83^{+0.09}_{-0.08}$, $1.85^{+0.41}_{-0.41}$ and $3.4^{+0.8}_{-0.7}$, respectively. This is inconsistent with the pressure equilibrium expected of a cold front and we conclude that the edge is a shock front. We measure a Mach number $M = 1.59^{+0.06}_{-0.07}$ and corresponding shock velocity $v_{rm shock}simeq 2057,$km/s. Using spectra collected at the MMT with the Hectospec multi-object spectrograph we identify 328 spectroscopically confirmed cluster members. Significantly, we find a local peak in the projected galaxy density associated with a bright cluster galaxy which is located just ahead of the nose of the shock. The data are consistent with a merger viewed within $sim 23,$degrees of the plane of the sky. The merging subclusters are now moving apart along a north-south axis approximately $0.3,$Gyr after a small impact parameter core passage. The gas core of the secondary subcluster, which was driving the shock, appears to have been disrupted by the merger. Without a driving piston we speculate that the shock is dying. Finally, we propose that the diffuse radio emission near the shock is due to the revival of pre-existing radio plasma which has been overrun by the shock.

Shocking Tails in the Major Merger Abell 2744

We identify four rare jellyfish galaxies in Hubble Space Telescope imagery of the major merger cluster Abell 2744. These galaxies harbor trails of star-forming knots and filaments which have formed in-situ in gas tails stripped from the parent galaxies, indicating they are in the process of being transformed by the environment. Further evidence for rapid transformation in these galaxies comes from their optical spectra, which reveal starburst, poststarburst and AGN features. Most intriguingly, three of the jellyfish galaxies lie near ICM features associated with a merging Bullet-like subcluster and its shock front detected in Chandra X-ray images. We suggest that the high pressure merger environment may be responsible for the star formation in the gaseous tails. This provides observational evidence for the rapid transformation of galaxies during the violent core passage phase of a major cluster merger.