3.4. X-Ray Emission Associated with Distant Galaxies
Most normal galaxies are rather weak X-ray emitters, with spatially extended X-ray emission in the range of ~ 1038 to ~ 1042 ergs s-1 (Fabbiano 1989). This emission largely derives from the hot phase of the interstellar medium (in early-type galaxies), close accreting binaries, and the end products of stellar evolution. Stellar coronal emission does not contribute significantly to the total X-ray output of normal galaxies. Extremely sensitive X-ray surveys, such as the 56 ks Röntgensatellit (ROSAT) Deep Survey in the Marano field (Zamorani et al. 1999) and the 1.5 Msecs ROSAT Deep Survey in the Lockman Hole (Hasinger et al. 1998; Schmidt et al. 1998), resolve most of the hard X-ray (0.5-2 keV) background into discrete sources. The majority of optically identified X-ray sources are broad-lined AGNs, i.e., quasars and Seyfert II galaxies. Recent claims of a new population of X-ray-emitting, narrow emission line galaxies dominating faint X-ray counts (Georgantopoulos et al. 1996) are not supported by the deepest surveys (Hasinger et al. 1998).
Thus, X-ray surveys are not expected to be significant tools for identifying normal, distant galaxies, although they can identify distant AGNs. The most distant X-ray source known currently is the radio-loud quasar GB 1428+4217 at z = 4.72 identified because of its X-ray emission (Fabian et al. 1997).
X-rays are also emitted by bremsstrahlung radiation from the hot gas in intracluster media. Therefore, spatially extended X-ray sources have considerable value in locating distant, overdense regions of the universe and are thus valuable probes for the formation and evolution of large-scale structure. Eke et al. (1996) demonstrate that the evolution of the abundance of rich clusters is strongly dependent upon the cosmological density parameter 0. Most X-ray clusters identified to date lie at z 0.5. Stanford et al. (1997) and Rosati et al. (1999) report two spatially proximate X-ray clusters at z 1.27.
The Chandra X-Ray Observatory (formerly known as AXAF) and X-Ray Multi-Mirror Mission (XMM) will soon extend X-ray detections to lower flux limits, providing a valuable aid in the detection of very distant AGN and galaxy clusters.