Far - infrared emission of intracluster dust in the Coma galaxy cluster ?

The ISOPHOT C200 camera aboard ISO has been used to observe the extended far infrared (FIR) emission from the Coma cluster of galaxies. Two scans with 48′ length across the cluster, each at 120 μm and 185 μm, were obtained at crossing position angles. The profiles of the surface brightness ratio I120μm/I185μm along the two scans are quite similar in showing an enhanced 120 μm emission within the central region of ≈ 10′ (0.4 Mpc) diameter. This flux excess is interpreted as thermal emission from intracluster dust with a temperature slightly higher than the galactic foreground cirrus. A conservative value for the excess surface brightness at 120 μm is ≈ 0.1 MJy/sr, corresponding to a total net flux of ≈ 0.7 Jy. Depending on the dust opacity and temperature, a total dust mass of 6.2×107M <∼MD <∼ 1.6×109M is inferred. The detected excess emission represents the first direct evidence from FIR observation for intracluster dust in a galaxy cluster. Since dust is destroyed effectively by the hot intracluster medium, the dust detected may result from galaxy stripping during the ongoing merging process in Coma. FIR emission from intracluster dust thusmight be used as a tool to search for non-equilibriumgalaxy clusters.