X-Ray Number Counts of Normal Galaxies

We use the number counts of X-ray-selected normal galaxies to explore their evolution by combining the most recent wide-angle shallow and pencil-beam deep samples available. The differential X-ray number counts, dN/dS, for early- and late-type normal galaxies are constructed separately and then compared with the predictions of the local X-ray luminosity function under different evolution scenarios. The dN/dS of early-type galaxies is consistent with no evolution out to z ≈ 0.5. For late-type galaxies, our analysis suggests that it is the sources with an X-ray-to-optical flux ratio log > -2 that are evolving the fastest. Including these systems in the late-type galaxy sample yields evolution of the form ≈(1 + z)2.7 out to z ≈ 0.4. On the contrary, late-type sources with log < -2 are consistent with no evolution. This suggests that the log > -2 population comprises the most powerful and fast-evolving starbursts at moderate and high z. We argue that although residual low-luminosity AGN contamination may bias our results toward stronger evolution, this is unlikely to modify our main conclusions.