ACTIVE GALACTIC NUCLEI AND STAR-FORMING GALAXIES - CONTRIBUTING TO THE EXTRAGALACTIC X-RAY FLUX

The nature of discrete sources contributing to the extragalactic 'X-ray background' (XRB) in the energy range approximately 0.1-3 keV has been investigated via optical identifications based on positional coincidence and spectroscopy. The results presented here address the counterparts to the discrete X-ray sources detected in the Einstein Observatory deep X-ray survey in Pavo. X-ray selected quasars, detected to a limiting flux of approximately 10(-14) erg cm-2 s-1 and a limiting B(J) magnitude of 21.5, have a surface density of approximately 32 per square degree, an average redshift of 0.94, and account directly for approximately 15 per cent of the extragalactic X-ray background at an effective energy of 2 keV. Other discrete-source detections in the Pavo survey have included the discovery of the first examples of star-forming (narrow emission line) galaxies at moderate redshift, of the kind which may contribute substantially to the XRB in the energy range approximately 3-20 keV. One further object is an elliptical (cD) galaxy in a cluster with X-ray emission possibly arising from a cooling flow.