The Origin Of The Cosmic Soft X-Ray Background: Optical Identification Of An Extremely Deep Rosat Survey

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was made with the ROSAT PSPC and reaches a flux limit of 1:6 \Theta 10 \Gamma15 erg cm \Gamma2 s \Gamma1 (0.5-2.0 keV). Above a flux limit of 2 \Theta 10 \Gamma15 erg cm \Gamma2 s \Gamma1 we define a complete sample of 70 sources of which 59 are identified. For a further 5 sources we have tentative identifications and for a further 4 the X-ray error-boxes are blank to R=23 mag. At brighter fluxes ( 10 \Gamma14 erg cm \Gamma2 s \Gamma1 ) we confirm the results of previous less deep X-ray surveys with 84% of our sources being QSOs. However at the faint flux limit the survey is dominated by a population of galaxies with narrow emission lines (NELGs). In addition at intermediate fluxes we find a small number of groups and clusters of galaxies at redshifts generally ? 0:3. Most of these groups are poor systems of low X-ray luminosity and the number which we find is consistent..