The origin of the cosmic soft X-ray background: optical identification of an extremely deep ROSAT survey

We present the results of the deepest optically identified X-ray survey yet made. The X-ray survey was obtained with the ROSAT position-sensitive proportional counter (PSPC) and reaches a flux limit of 1.6 x 10(-15) erg cm(-2) s(-1) (0.5-2.0 keV). Above a flux limit of 2 x 10(-15) erg cm(-2) s(-1) we define a complete sample of 70 sources, of which 59 are identified. For a further five sources we have tentative identifications and for a further four the X-ray error boxes are blank to R = 23 mag. At brighter fluxes (greater than or equal to 10(-14) erg cm(-2) s(-1)) we confirm the results of previous less deep X-ray surveys, with 84 per cent of our sources being QSOs. However, at the faint flux limit the survey is dominated by a population of galaxies with narrow emission lines (NELGs). In addition, at intermediate fluxes we find a small number of groups and clusters of galaxies at redshifts generally >0.3. Most of these groups are poor systems of low X-ray Luminosity and the number that we find is consistent with a zero evolutionary scenario, unlike the situation for high-luminosity clusters at the same redshift. To a flux limit of 2 x 10(-15) erg cm(-2) s(-1), QSOs contribute > 31 per cent of the cosmic soft X-ray background (XRB), groups/clusters contribute similar to 10 per cent and NELGs contribute similar to 8 per cent. However, the QSO differential source count slope below 10(-14) erg cm(-2) s(-1) is similar to-1.4, severely sub-Euclidean, as is the (poorly defined) group/cluster slope, whereas the differential NELG slope is close to Euclidean (similar to-2.4). If the NELG source counts continue to rise at that slope, all of the remaining cosmic soft XRB will be explained by a flux limit of similar to 1-2 x 10(-16) erg cm(-2) s(-1), with NELGs contributing about one quarter of the XRB. The average NELG X-ray spectrum is harder than that of the QSOs, and similar to that of the remaining unresolved cosmic XRB, suggesting that NELGs will also be substantial contributors to the XRB at higher energies. The observed NELGs lie in the redshift range 0.1-0.6 and have M-R = -20 to -23, approximately 3 mag more luminous than typical field galaxies. They have predominantly blue colours, and some are definitely spirals, but the presence of some ellipticals cannot yet be ruled out. Many are in interacting or disturbed systems. The NELGs have optical spectra similar to those of the majority of the field galaxy population at a similar redshift and may simply be the more luminous members of the emission-line field galaxy population. Based on optical line ratios and X-ray/optical flux ratios, the NELGs, both as a sample and within individual galaxies, appear to be a mixture of starburst galaxies and true active galactic nuclei (AGN).