The ROSAT/IRAS galaxy sample revisited

Galaxies in the ROSAT/IRAS sample were selected by their soft X-ray (0.1-2.4 keV) and far-infrared (lambda = 60 mu m) 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-flux" (S > 10 Jy at lambda = 100 mu m) IRAS sources from the original sample. We used new 1.4 GHz VLA images of all objects north of delta = -45 degrees along with improved X-ray and far-infrared positions to eliminate incorrect identifications, 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 identifications among the "high-flux" sources with delta > -45 degrees. 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-ray-emitting active galactic nuclei (AGNs) residing in star-forming disks that emit most of the lambda = 60 mu m radiation. Normal and starburst galaxies probably do not account for a significant fraction of the soft X-ray background.