The weak outnumbering the mighty: normal galaxies in deep Chandra surveys

Chandra is detecting a significant population of normal and starburs t galaxies in extremely deep X-ray exposures. For example, approximately 15% of the sources arising in the 2 Ms Chandra Deep Field-North survey are fairly normal galaxies, where "normal" means "Milky Way-type" X-ray emission rather than simply exhibiting an "optically normal" spectrum. Many of these galaxies are being detected at large look-back times (z � 0.1-0.5), allowing the study of the evolution of X-ray binary populations over significant cosm ological timescales. We are also detecting individual off- nuclear ultraluminous X-ray sources (e.g., X-ray binaries), provi ding the first direct constraints on the prevalence of lower- mass black holes at significantly earlier times. The X-ray emission fro m such "normal" galaxies may also be a useful star-formation rate indicator, based on radio/X-ray cross-identifications. We describe the contribution of normal galaxies to the populations which make up the X-ray background and present their directly measured X-ray number counts. We find that normal and starburst galaxies should dominate the 0.5-2 keV number counts at X-ray fluxes fainter than � 7×10 18 erg cm 2 s 1 (thus they will outnumber the "mighty" AGN). Finally, we look to the future, suggesting that it is important that the populatio n of X-ray faint normal and starburst galaxies be well constrained in order to design the next generation of X-ray observatories.