NASA/IPAC EXTRAGALACTIC DATABASE
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For refcode 1996ApJS..106..341M:
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Copyright by American Astronomical Society. Reproduced by permission
1996ApJS..106..341M CLASSIFICATION OF IRAS-SELECTED X-RAY GALAXIES IN THE ROSAT ALL-SKY SURVEY EDWARD C. MORAN, JULES P. HALPERN, AND DAVID J. HELFAND Department of Astronomy, Columbia University, 538 West 120th Street, New York, NY 10027 Received 1995 April 6; accepted 1996 March 15 ABSTRACT To explore the possibility that star-forming galaxies or obscured Seyfert galaxies, both of which are known to be luminous infrared sources, contribute significantly to the cosmic X-ray background, we have carried out an extensive program to obtain accurate spectroscopic classifications of the Boller et al. (1992) catalog of IRAS sources detected in the ROSAT All-Sky Survey. This has involved careful optical spectroscopy, a review of the literature, and efforts to reveal the contaminants 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 and have established strong cases for 29 others. Most chance coincidences involve bright stars or Seyfert galaxies close (in projection) to IR- 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 (AGNs), 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 types of AGNs are found in this sample with surprising frequency: I Zw 1 objects (also called narrow-line Seyfert 1 galaxies) and starburst/Seyfert composite galaxies, a new class of luminous X-ray sources. We have shown that the Boller 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 composite galaxies have optical spectra dominated by the features of H II galaxies but X-ray luminosities typical for Seyfert galaxies. Close examination of their optical spectra reveals subtle Seyfert signatures: [O III] lines broader than all other lines in the spectrum and, in some cases, a weak, broad H{alpha} 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 AGNs could comprise an important new component of the X-ray background. Subject headings: galaxies: active - galaxies: Seyfert - infrared: galaxies - X-rays: galaxies
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