IC 1613 X-l: A Galaxy Cluster at z≈0.2 Behind a Local Group Dwarf Galaxy

摘要

Based on Einstein IPC imaging and spectroscopy, optical spectroscopy, and optical imaging, I argue that the X rays seen in the direction of the Local Group dwarf irregular galaxy IC 1613 are mainly due to emission from a previously uueataloged cluster of galaxies (CL B0102+0146) at a redshift of z=*0.2 and of richness class 3, centered at the IPC X-ray position. The Einstein data are not adequate to determine if the X rays are due to cluster emission or an AGN. However, both the available optical data, and the nondetection of this object in deep radio surveys argue that a hot intracluster medium is the source of the X-ray emission. The X-ray Jlux of this cluster in the Einstein IPC band (0.2-4.0 keV) is higher than 78% of the clusters in the Einstein Hxtended Medium Sensitivity Survey sample of Gioiaet al. (1990). It is thus likely that only its position behind IC 1613 prevented its earlier identification.