ITAL]Chandra[/ITAL] Estimate of the Magnetic Field Strength near the Cold Front in A3667

Abstract

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 through the less dense, hotter gas. Without a magnetic field, the front should be quickly disturbed by the Kelvin-Helmholtz instability arising from tangential motion of gas layers. However, the Chandra image shows that the front is stable within a ±30° sector in the direction of the cloud motion, beyond which it gradually disappears. We suggest that the Kelvin-Helmholtz instability within the ±30° sector is suppressed by the surface tension of the magnetic field whose field lines are parallel to the front. The required field strength is B ~ 10 μG. The magnetic field near the front is expected to be stronger and to have a very different structure compared with the bulk of the intergalactic medium because the field lines are stretched by the tangential gas motions. Such a magnetic configuration, once formed, would effectively stop the plasma diffusion and heat conduction across the front and may inhibit gas mixing during the subcluster merger. We note that even the increased magnetic field near the front contributes only 10%-20% to the total gas pressure, and therefore magnetic pressure is unimportant for hydrostatic cluster mass estimates.