A deep ROSAT survey - XIV. X-ray emission from faint galaxies

We present a cross-correlation analysis to constrain the faint galaxy contribution to the cosmic X-ray background (XRB). Cross-correlating faint optical galaxy catalogues with unidentified X-ray sources from three deep ROSAT fields, we find that B<23 galaxies account for 20+/-7 per cent of all X-ray sources to a flux limit of S(0.5-2.0 keV)=4x10^-15 erg s^-1 cm^-1. To probe deeper, galaxies are then cross-correlated with the remaining unresolved X-ray images. A highly significant signal is obtained on each field. Allowing for the effect of the ROSAT point-spread function, and deconvolving the effect of galaxy clustering, we find that faint B<23 galaxies directly account for 23+/-3 per cent of the unresolved XRB at 1keV. Using the optical magnitude of faint galaxies as probes of their redshift distribution, we find evidence for strong evolution in their X-ray luminosity, parametrized with the form L_x~(1+z)^3.2+/-1.0. Extrapolation to z=2 will account for 40+/-10 per cent of the total XRB at 1keV. The nature of the emitting mechanism in these galaxies remains unclear, but we argue that obscured and/or low-luminosity AGN provide the most plausible explanation.