Chandra Temperature Maps for Galaxy Clusters with Radio Halos

This presentation is based on the paper by Govoni et al. (2004). We derived Chandra temperature maps for a sample of clusters with high-quality radio halo data, to study the origin of the radio halos. All the sample clusters exhibit distorted X-ray morphology and strong gas temperature variations indicating ongoing mergers. Some clusters, e.g., A520, A665, 1E0657 56, exhibit spatial correlation between the radio halo brightness and the hot gas regions. However, it is not a general feature. We find clear counterexamples (e.g., A754 and A773) where 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 1E0657 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 1E0657 56, A520 and A665 (all three high-velocity mergers) extend into the shock regions in front of the subclusters, where no strong turbulence is expected. Thus, in high-velocity ( 2–3) mergers, both shock and turbulence acceleration mechanisms may be significant.