A deep ROSAT survey .5. The extragalactic populations at faint fluxes

In the central regions of five deep (21-49 ks) ROSAT Position Sensitive Proportional Counter (PSPC) pointings we have detected 194 soft X-ray sources down to a limiting flux of similar to 3x10(-15) erg cm(-2) s(-1) (0.5-2 keV). Spectroscopic follow-up observations at the Anglo-Australian Telescope (AAT) show that the majority of these sources are active galactic nuclei (AGN) (QSOs and Seyfert 1 galaxies); some X-ray luminous galaxies (L(X) similar to 10(42) erg s(-1)) are also identified at faint fluxes; these may have active nuclei or they may be star-forming galaxies, The intrinsic spectrum of the total X-ray background, including sources, is well fitted above 1 keV by a power-law model with a photon index of Gamma similar to 1.6, flatter than the average AGN spectrum in our fields. The normalization of this component at 1 keV is some 50 per cent higher than an extrapolation of the X-ray background (XRB) spectrum measured above 3 keV. This extends the XRB spectral paradox to lower energies than has previously been noted, and suggests that some population other than QSOs and Seyfert 1 galaxies may contribute substantially to the XRB at faint fluxes. This is supported by an analysis of the source number count distribution (logN-logS): the differential logN-logS of AGN and unidentified objects becomes a flat power law (gamma approximate to 1.1+/-0.9, 90 per cent error) at S similar to 10(-14) erg cm(-2) s(-1), yielding a contribution of only 45 per cent at the 1-keV background when integrated down to 10(-16) erg cm(-2) s(-1); despite the large log N-log S uncertainty at faint fluxes the hypothesis that AGN produce all the soft XRB intensity can be ruled out at the 90 per cent confidence level. Galaxies such as those we have detected will contribute 20 per cent of the 1-keV background if they follow a Euclidean logN-logS distribution.