ar X iv : a st ro - p h / 03 07 37 0 v 1 2 1 Ju l 2 00 3 Chandra Observations of Point Sources in Abell 2255

In our search for “hidden” AGN we present results from a Chandra observation of the nearby cluster Abell 2255. Eight cluster galaxies are associated with point-like X-ray emission, and we classify these galaxies based on their X-ray, radio, and optical properties. At least three are associated with active galactic nuclei (AGN) with no optical signatures of nuclear activity, with a further two being potential AGN. Of the potential AGN, one corresponds to a galaxy with a post-starburst optical spectrum. The remaining three X-ray detected cluster galaxies consist of two starbursts and an elliptical with luminous hot gas. Of the eight cluster galaxies five are associated with luminous (massive) galaxies and the remaining three lie in much lower luminosity systems. We note that the use of X-ray to optical flux ratios for classification of X-ray sources is often misleading, and strengthen the claim that the fraction of cluster galaxies hosting an AGN based on optical data is signficantly lower than the fraction based on X-ray and radio data. Subject headings: galaxies: clusters: individual (Abell 2255) — galaxies: clusters: general — galaxies: active — galaxies: intergalactic medium — X-rays: galaxies Laboratory for High Energy Astrophysics, Code 661, Greenbelt, MD 20771 Joint Center for Astrophysics, Department of Physics, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250 NASA/GSFC UV/Optical Branch, Code 681, Greenbelt, MD 20771 Laboratory for High Energy Astrophysics, Code 662, Greenbelt, MD 20771