The weak outnumbering the mighty: normal galaxies in deep Chandra surveys
Abstract
Chandra is detecting a significant population of normal and starburst galaxies in extremely deep X-ray exposures. For example, approximately 15\ 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*E-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.
- Publication:
-
Astronomische Nachrichten
- Pub Date:
- 2003
- DOI:
- 10.1002/asna.200310004
- arXiv:
- arXiv:astro-ph/0211487
- Bibcode:
- 2003AN....324...12H
- Keywords:
-
- surveys;
- X-rays: galaxies;
- X-rays: black holes;
- Astrophysics
- E-Print:
- Proceedings for the "X-ray Surveys: in the light of new observatories" workshop (Astron. Nachrichten, in press