SIMBAD references

We analyze Chandra temperature maps for a sample of clusters with high-quality radio halo data to study the origin of the radio halos. The sample includes A520, A665, A754, A773, A1914, A2163, A2218, A2319, and 1E 0657-56. We present new temperature maps for all but two of them (A520 and A754). All these clusters exhibit distorted X-ray morphology and strong gas temperature variations indicating ongoing mergers. Some clusters, e.g., A520, A665, 1E 0657-56, exhibit the previously reported spatial correlation between the radio halo brightness and the hot gas regions. However, it is not a general feature. While most mergers are too messy to allow us to disentangle the projection effects, we find clear counterexamples (e.g., A754 and A773) in which the hottest gas regions do not exhibit radio emission at the present sensitivity level. This cannot be explained by projection effects and therefore argues against merger shocks–at least those relatively weak ones responsible for the observed temperature structure in most clusters–as the main mechanism for the halo generation. This leaves merger-generated turbulence as a more likely mechanism. The two clusters with the clearest radio brightness-temperature correlation, A520 and 1E 0657-56, are both mergers in which a small dense subcluster has just passed through the main cluster, very likely generating turbulence in its wake. The maximum radio brightness and the hot gas are both seen in these wake regions. On the other hand, the halos in 1E 0657-56 and A665 (both high-velocity mergers) extend into the shock regions in front of the subclusters, where no strong turbulence is expected. Thus, in high-velocity (M≃2-3) mergers, both shock and turbulence acceleration mechanisms may be significant.