CLASSIFICATION OF IRAS-SELECTED X-RAY GALAXIES IN THE ROSAT ALL-SKY SURVEY

To explore the possibility that star-forming galaxies or obscured Seyf ert galaxies, both of which are known to be luminous infrared sources, contribute significantly to the cosmic X-ray background, we have carr ied out an extensive program to obtain accurate spectroscopic classifi cations of the Boller et al. (1992) catalog of IRAS sources detected i n the ROSAT All-Sky Survey. This has involved careful optical spectros copy, a review of the literature, and efforts to reveal the contaminan ts in the sample. Classifications have been determined for 210 of the 241 X-ray sources in the catalog; 105 are presented here for the first time. A large number of IR/X-ray source chance coincidences are found in this sample; of the 40-50 expected, we have identified 18 firmly a nd have established strong cases for 29 others. Most chance coincidenc es involve bright stars or Seyfert galaxies close (in projection) to I R-bright H II galaxies. Although this work was motivated initially by the report that a new class of X-ray-luminous, normal spiral galaxies was to be found in this sample, we find no evidence for such a class. Most of the extragalactic X-ray sources are active galactic nuclei (AG Ns), consistent with the results of previous studies of X-ray-selected objects. However, many of these AGNs exhibit weak or heavily reddened Seyfert features in their optical spectra. In addition, two rare type s of AGNs are found in this sample with surprising frequency: I Zw 1 o bjects (also called narrow-line Seyfert 1 galaxies) and starburst/Seyf ert composite galaxies, a new class of luminous X-ray sources. We have shown that the Boiler et al. object 202103-223434 (=IRAS 20181-2244), reported to be the best example of a narrow-line quasar, is actually a member of the I Zw 1 class. The enigmatic starburst/Seyfert composit e galaxies have optical spectra dominated by the features of H II gala xies but X-ray luminosities typical for Seyfert galaxies. Close examin ation of their optical spectra reveals subtle Seyfert signatures: [O I II] lines broader than all other lines in the spectrum and, in some ca ses, a weak, broad Her component. Obscuration of the active nucleus is likely to explain the X-ray and optical properties of these objects. We describe a scenario in which such optically innocuous, obscured AGN s could comprise an important new component of the X-ray background.