A ROSAT deep survey – VI. Active and passive X-ray galaxies

We have shown previously that ROSAT deep-survey X-ray sources (to limiting fluxes of ~ 10–14 erg cm–2 s–1, 0.5–2 keV) are predominantly identified with active galactic nuclei, but this paper focuses on the smaller fraction of objects identified with emission-line galaxies and late-type ‘passive’ galaxies. The nature of the discrete X-ray sources has been directly investigated by means of multi-object spectroscopy of candidate optical counterparts based on positional coincidence to within ~ 15 arcsec. A small number of early-type of ‘passive’ galaxies are detected within 300 km s–1 of a common redshift of 0.312, and may be members of a large (14 Mpc) structure. There is no clear evidence for extended intracluster gas, however, and there is no X-ray or optical evidence for a rich cluster. Several possibilities as to the origin of the X-ray emission from these passive galaxies are discussed: (i) active galactic nuclei within otherwise passive galaxies (although no emission lines have been observed); (ii) hot coronae of elliptical galaxies, possibly in the early stages of cooling flows; and (iii) small groups of galaxies, or ‘fossil’ group emission, centred on the elliptical. Other discrete source detections have included the first examples of starburst and star-forming (narrow emission-line) galaxies in ROSAT deep surveys, of the kind which may also contribute to the XRB in the ROSAT energy range.