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

To investigate the contribution of faint galaxies to the ~1-keV X-ray background (XRB), we cross-correlate the positions of 2750 galaxies with 18<=R<=23 mag, detected on optical CCD images, with the unresolved fluctuations on a deep (74 ks) ROSAT PSPC image. We detect a positive and significant (5sigma) signal in the cross-correlation function, in good agreement with our previous results from slightly less deep data. We measure similar cross-correlation amplitudes for brighter (18<=R<21) and fainter (21<=R<=23) subsets of the galaxies. These results are consistent with a galaxy X-ray emissivity of rho_0(0.7-2.0 keV)=1.09+/-0.14x10^38h_50 erg s^-1 Mpc^-3 locally, increasing with AGN-like evolution to rho_z(0.7-2.0 keV)~=4.16+/-0.53x10^38h_50 erg s^-1 Mpc^-3 at the mean redshift of the galaxies, and would indicate that the faint galaxy contribution to the XRB is similar to that from QSOs. The galaxies show cross-correlations similar to the unresolved XRB in the 0.70-1.07 and 1.07-2.0 keV energy bands. From the ratio of these two cross-correlations, we derive an X-ray hardness ratio for the summed emission from 18<=R<=23 galaxies. We find this corresponds to either a high-temperature (<e1>kTd<~</e1>2.5 keV) thermal spectrum or a power law with spectral index alpha_x~=-0.6+/-0.7. This is harder than the expected emission from giant ellipticals, but in agreement with the X-ray properties of star-forming galaxies. Extrapolation of our results to higher redshifts suggests that evolving, star-forming galaxies would produce ~30-50 per cent of the total 0.7-2.0 keV XRB, sufficient to explain the non-QSO component of the XRB in this energy range.