A Merger Shock in Abell 2034

Abstract

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 $1.83_{-0.08}^{+0.09}$, $1.85_{-0.41}^{+0.41}$ and $3.4_{-0.7}^{+0.8}$, 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 $M=1.59_{-0.07}^{+0.06}$ and corresponding shock velocity $v_{\mathrm{shock}}\simeq 2057$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.

14 pages, 8 figures, 3 tables. Accepted for publication in ApJ

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