Chandra estimate of the magnetic field strength near the cold front in A3667

We use the Chandra observation of the cold front in the intracluster gas of A3667 to estimate the magnetic field strength near the front. The front is seen in the Chandra data as a sharp discontinuity in the gas density that delineates a large body of dense cool gas moving with near-sonic velocity t hrough the less dense, hotter gas. Without a magnetic field, the front shou ld be quickly disturbed by the Kelvin-Helmholtz instability arising from ta ngential motion of gas layers. However, the Chandra image shows that the fr ont is stable within a +/-30 degrees sector in the direction of the cloud m otion, beyond which it gradually disappears. We suggest that the Kelvin-Hel mholtz instability within the +/-30 degrees sector is suppressed by the sur face tension of the magnetic field whose field lines are parallel to the fr ont. The required field strength is B similar to 10 muG. The magnetic field near the front is expected to be stronger and to have a very different str ucture compared with the bulk of the intergalactic medium because the field lines are stretched by the tangential gas motions. Such a magnetic configu ration, once formed, would effectively stop the plasma diffusion and heat c onduction across the front and may inhibit gas mixing during the subcluster merger. We note that even the increased magnetic field near the front cont ributes only 10%-20% to the total gas pressure, and therefore magnetic pres sure is unimportant for hydrostatic cluster mass estimates.