A moving cold front in the intergalactic medium of A3667

We present results from a Chandra observation of the central region of the galaxy cluster A3667 with emphasis on the prominent sharp X-ray brightness edge spanning 0.5 Mpc near the cluster core. Our temperature map shows larg e-scale nonuniformities characteristic of the ongoing merger, in agreement with earlier ASCA results. The brightness edge turns out to be a boundary o f a large cool gas cloud moving through the hot ambient gas, very similar t o the "cold fronts" discovered by Chandra in A2142. The higher quality of t he A3667 data allows the direct determination of the cloud velocity. At the leading edge of the cloud, the gas density abruptly increases by a factor of 3.9 +/- 0.8, while the temperature decreases by a factor of 1.9 +/- 0.2 (from 7.7 to 4.1 keV). The ratio of the gas pressures inside and outside th e front shows that the cloud moves through the ambient gas at near-sonic ve locity, M = 1 +/- 0.2 or v = 1400 +/- 300 km s(-1). In front of the cloud, we observe the compression of the ambient gas with an amplitude expected fo r such a velocity. A smaller surface brightness discontinuity is observed f urther ahead, similar to 350 kpc in front of the cloud. We suggest that it corresponds to a weak bow shock, implying that the cloud velocity may be sl ightly supersonic. Given all the evidence, the cold front appears to deline ate the remnant of a cool subcluster that recently has merged with A3667. T he cold front is remarkably sharp. The upper limit on its width, 3."5 or 5 kpc, is several times smaller than the Coulomb mean free path. This is a di rect observation of suppression of the transport processes in the intergala ctic medium, most likely by magnetic fields.