The [ITAL]ROSAT[/ITAL]/[ITAL]IRAS[/ITAL] Galaxy Sample Revisited

Galaxies in the ROSAT /IRAS sample were selected by their soft X-ray (0.12.4 keV) and far-infrared (j \ 60 km) emission. The relatively large uncertainties in the original ROSAT and IRAS positions caused some contamination by close pairs and forced the exclusion of most "" high-ux (S [ 10 Jy at j \ 100 km) IRAS sources from the original sample. We used new 1.4 GHz VLA images of all objects north of d \ [45 along with improved X-ray and far-infrared positions to eliminate incorrect identications, many of which appeared to be starburst galaxies with high X-ray luminosities, log [X(ergs~1)] [ 43. We also used VLA images to search for new X-ray identications among the "" high-ux sources with d [ [45. Only two were found, indicating that luminous starburst galaxies have relatively low soft X-ray luminosities, in part due to absorption by a dense interstellar medium. No starburst galaxies in our revised sample have X-ray luminosities approaching log [X(ergs~1)] \ 43. We conclude that most galaxies in the revised ROSAT /IRAS sample contain X-rayemitting active galactic nuclei (AGNs) residing in star-forming disks that emit most of the j \ 60 km radiation. Normal and starburst galaxies probably do not account for a signicant fraction of the soft X-ray background.