A deep ROSAT survey .11. Enhanced X-ray emission from faint galaxies

To investigate the contribution of faint galaxies to the similar to 1-keV X-ray background (XRB), we cross-correlate the positions of 2750 galaxies with 18 less than or equal to R less than or equal to 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 (5 sigma) signal in the crosscorrelation function, in good agreement with our previous results from slightly less deep data. We measure similar cross-correlation amplitudes for brighter (18 less than or equal to R < 21) and fainter (21 less than or equal to R less than or equal to 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 +/- 0.14 x 10(38) h(50) erg s(-1) Mpc(-3) locally, increasing with AGN-like evolution to rho(z)(0.7-2.0 keV) similar or equal to 4.16 +/- 0.53 x 10(38) h(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 crosscorrelations, we derive an X-ray hardness ratio for the summed emission from 18 less than or equal to R less than or equal to 23 galaxies. We find this corresponds to either a high-temperature (kT > 2.5 keV) thermal spectrum or a power law with spectral index alpha(x) similar or equal to - 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 similar to 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.