A deep ROSAT survey - VII. A faint galaxy contribution to the X-ray background

Previously, we have determined by direct detection that QSOs account for ~=30 per cent of the X-ray background at 1 keV. There are indications, however, that QSOs do not produce the entire X-ray background. Using deep ROSAT PSPC observations in five fields where we also have deep optical imaging data, we perform optical spectroscopic follow-up investigation of faint ROSAT sources. By directly cross-correlating these X-ray sources with galaxies, we estimate that galaxies to B=21 mag form ~5 per cent of the detected X-ray sources. We detect a highly significant (>5 alpha) cross-correlation between B<23 mag galaxies observed on three AAT plates and the fluctuations in the unresolved 0.5-2.0 keV X-ray background on the deep ROSAT images. The amplitude of this cross-correlation appears to indicate that galaxies to B=23 mag produce 16.8+/-2.3 per cent of the unresolved X-ray background. Extrapolation of these results to higher redshifts would give a total contribution of faint galaxies to the X-ray background which is comparable to that of QSOs.