Active galactic nuclei and star-forming galaxies - contributing to the extragalactic X-ray flux

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Abstract
The nature of discrete sources contributing to the extragalactic ‘X-ray background’ (XRB) in the energy range ~ 0.1–3 keV has been investigated via optical identifications based on positional coincidence and spectroscopy. The results presented 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 |$\sim{10}^{-14}\,\text{erg}\,{\,\text{cm}}^{-2}{\text{s}}^{-1}$| and a limiting BJ magnitude of 21.5, have a surface density of ~ 32 per square degree, an average redshift of 0.94, and account directly for ~ 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 ~ 3–20 keV. One further object is an elliptical (cD) galaxy in a cluster with X-ray emission possibly arising from a cooling flow.

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