[ITAL]Chandra[/ITAL] Observation of a 300 Kiloparsec Hydrodynamic Instability in the Intergalactic Medium of the Merging Cluster of Galaxies A3667

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Abstract
We present results from the combination of two Chandra pointings of the central region of the cluster of galaxies A3667. From the data analysis of the first pointing, Vikhlinin, Markevitch, and Murray reported the discovery of a prominent cold front that is interpreted as the boundary of a cool gas cloud moving through the hotter ambient gas. They discussed the role of the magnetic fields in maintaining the apparent dynamical stability of the cold front over a wide sector at the forward edge of the moving cloud and in suppressing transport processes across the front. In this Letter, we identify two new features in the X-ray image of A3667: (1) a 300 kpc arclike filamentary X-ray excess extending from the cold gas cloud border into the hotter ambient gas and (2) a similar arclike filamentary X-ray depression that develops inside the gas cloud. Both features are located beyond the sector identified by the cold front and are oriented in a direction perpendicular to the direction of motion. The temperature map suggests that the temperature of the filamentary excess is consistent with that inside the gas cloud, while the temperature of the depression is consistent with that of the ambient gas. We suggest that the observed features represent the first evidence for the development of a large-scale hydrodynamic instability in the cluster atmosphere resulting from a major merger. This result confirms previous claims for the presence of a moving cold gas cloud in the hotter ambient gas. Moreover, it shows that, although the gas mixing is suppressed at the leading edge of the subcluster as a result its magnetic structure, strong turbulent mixing occurs at larger angles toward the direction of motion. We show that this mixing process may favor the deposition of a nonnegligible quantity of thermal energy right in the cluster center, affecting the development of the central cooling flow.