THE X-RAY BACKGROUND AND THE STARFORMING HISTORY IN THE UNIVERSE

                        Guenther Hasinger (AIP)

X-ray surveys using ROSAT, ASCA and BeppoSAX have shown that the cosmic X-ray 
background (XRB) is largely due to accretion onto supermassive black holes, 
integrated over cosmic time. The deep X-ray surveys detect a larger surface 
density of AGN than in any other waveband and a space density that evolves as 
steeply as (1+z)^5 for high-luminosity AGN out to a redshift of 2 and is 
consistent with a constant value at higher redshift. These findings support the
notion that most larger galaxies contain black holes which have been formed 
early and have been active in the past. However, the characteristic hard 
spectrum of the XRB can only be explained if most AGN spectra are heavily 
absorbed. Thus as much as 90% of the light produced by accretion may be 
absorbed by gas and dust clouds, which could reside in nuclear starburst 
regions feeding the AGN.

This scenario has important consequences for the current attempts to understand
black hole and galaxy formation and evolution: first, the absorbed AGN will 
suffer severe extinction and therefore, unlike classical QSOs, will not be 
prominent at optical wavelengths. Indeed, a number of X-ray luminous galaxies 
with only narrow emission lines in the optical band have been found in the deep
X-ray surveys that may well be obscured AGN. Secondly, since most of the 
accretion power in the universe is being absorbed by gas and dust, it will have
to be reradiated in the FIR range and be redshifted into the sub-mm band. AGN 
could therefore contribute a substantial fraction to the recently discovered 
cosmic FIR/sub-mm background which has already partly been resolved by deep 
SCUBA surveys in the Hubble Deep Field and the Lockman Hole. This contribution
has to be taken into account when discussing obscured star formation in the
early universe.