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

Abstract

AbstractChandra is detecting a significant population of normal and starburst 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 cosmological timescales. We are also detecting individual off‐nuclear ultraluminous X‐ray sources (e.g., X‐ray binaries), providing the first direct constraints on the prevalence of lower‐mass black holes at significantly earlier times. The X‐ray emission from 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 population of X‐ray faint normal and starburst galaxies be well constrained in order to design the next generation of X‐ray observatories.