X-Ray Number Counts of Normal Galaxies

We use the number counts of X-ray selected normal galaxies to explore theirevolution by combining the most recent wide-angle shallow and pencil-beam deepsamples available. The differential X-ray number counts, dN/dS, for early andlate-type normal galaxies are constructed separately and then compared with thepredictions of the local X-ray luminosity function under different evolutionscenarios. The dN/dS of early type galaxies is consistent with no evolution outto z~0.5. For late-type galaxies our analysis suggests that it is the sourceswith X-ray--to--optical flux ratio logfx/fopt>-2 that are evolving the fastest.Including these systems in the late-type galaxy sample yields evolution of theform ~(1+z)^{2.7} out to z~0.4. On the contrary late-type sources withlogfx/fopt<-2 are consistent with no evolution. This suggests that thelogfx/fopt>-2 population comprises the most powerful and fast evolvingstarbursts at moderate and high-z. We argue that although residuallow-luminosity AGN contamination may bias our results toward strongerevolution, this is unlikely to modify our main conclusions.Comment: to appear in ApJ