A ROSAT deep survey - VI. Active and passive X-ray galaxies
Abstract
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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- July 1995
- DOI:
- 10.1093/mnras/275.1.77
- Bibcode:
- 1995MNRAS.275...77G
- Keywords:
-
- GALAXIES: ELLIPTICAL AND LENTICULAR;
- CD;
- X-RAYS: GALAXIES;
- X-RAYS: GENERAL