FAR-INFRARED EMISSION OF INTRACLUSTER DUST IN THE COMA GALAXY CLUSTER

The ISOPHOT C200 camera aboard ISO has been used to observe the extend ed far infrared (FIR) emission from the Coma cluster of galaxies. Two scans with 48' length across the cluster, each at 120 mu m and 185 mu m, were obtained at crossing position angles. The profiles of the surf ace brightness ratio I-120 mu m/I-185 mu m along the two scans are qui te similar in showing an enhanced 120 mu m emission within the central region of approximate to 10' (0.4 Mpc) diameter. This flux excess is interpreted as thermal emission from intracluster dust with a temperat ure slightly higher than the galactic foreground cirrus. A conservativ e value for the excess surface brightness at 120 mu m is approximate t o 0.1 MJy/sr, corresponding to a total net flux of approximate to 0.7 Jy. Depending on the dust opacity and temperature, a total dust mass o f 6.2 x 10(7) M. less than or similar to M-D less than or similar to 1 .6 x 10(9) M. 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 intrac luster medium, the dust detected may result from galaxy stripping duri ng the ongoing merging process in Coma. FIR emission from intracluster dust thus might be used as a tool to search for non-equilibrium galax y clusters.