New Constraints on Galaxy Evolution from the Optical Monitor on XMM-Newton

Abstract

We use galaxies detected in a deep ultraviolet XMM-Newton Optical Monitor image and a model that predicts UV galaxy counts based on local counts and evolution parameters to constrain galaxy evolution to Z=1.2. The 17' square 2000 angstrom (UVW2 filter) image was taken as part of the XMM-OM team's guaranteed time program. We detect sources in this image to a flux limit of 2.7 X 10^{-17} ergs/cm^2/s/A, (AB magnitude = 22). Since some of the sources may be stars, we perform a number of checks, including shape, color and implied distance to remove stars from the detected counts. We find galaxy number counts as a function of magnitude roughly in agreement with previous studies, but again find these counts are in excess of evolution models. The excess counts at faint magnitudes may provide evidence for either a new population of galaxies emerging around Z=0.7 or more dramatic evolution than some earlier predictions. The integrated light from the detected galaxies totals 32--36 ph/cm^2/s/A/sr, placing a firm lower limit on the integrated UV light from galaxies.