FAINT GALAXIES AND THE X-RAY-BACKGROUND

We summarise our recent work on the faint galaxy contribution to the c os-c X-ray background (XRB). At bright X-ray fluxes (in the ROSAT pass band), broad line QSOs dominate the X-ray source population, but at f ainter fluxes there is evidence for a significant contribution from em ission-line galaxies. Here we present statistical evidence that these galaxies can account for a large fraction of the XRB. We also demonstr ate that these galaxies have significantly harder X-ray spectra than Q SOs in the ROSAT band. Finally we present preliminary findings from in fra-red spectroscopy on the nature of this X-ray emitting galaxy popul ation. We conclude that a hybrid explanation consisting of obscured/Ty pe 2 AGN surrounded by starburst activity can explain the properties o f these galaxies and perhaps the origin of the entire XRB.