OPTICAL SPECTROSCOPY OF THE IRAS 1-Jy SAMPLE OF ULTRALUMINOUS INFRARED GALAXIES

This paper discusses the optical spectroscopic properties of the IRAS 1-Jy sample (f60 > 1 Jy) of ultraluminous infrared galaxies (ULIGs: Lir > 10 12 L⊙; Ho = 75 km s −1 Mpc and qo = 0). One hundred and eight of the 118 1-Jy ULIGs have been observed at ∆λ = 8.3 Å resolution over the wavelength range ∼ 4500 Å – 8900 Å. These data are combined with large, previously published sets of optical spectroscopic data of lower luminosity infrared galaxies to look for systematic trends with infrared luminosity over the luminosity range Lir ≈ 10 10.5 − 10 L⊙. As found in previous studies, the fraction of Seyfert galaxies among luminous infrared galaxies increases abruptly above Lir ≈ 10 12.3 L⊙ — about 50% of the galaxies with Lir > 10 12.3 L⊙ present Seyfert characteristics. Many of the optical and infrared spectroscopic properties of the Seyfert galaxies are consistent with the presence of a genuine active galactic nucleus (AGN). About 30% of these galaxies are Seyfert 1s with broad-line regions similar to those of optical quasars. Published near-infrared spectroscopy also suggests that many of the Seyfert 2 galaxies (especially those with warm IRAS 25-to-60 μm colors) are in fact obscured Seyfert 1 galaxies with broad (∼> 2,000 km s ) recombination lines at 2 μm, where dust obscuration is less important. The percentage of Seyfert 1 ULIGs increases with infrared luminosity, contrary to the predictions of the standard unification model for Seyfert galaxies. Comparisons of the broad-line luminosities of optical and obscured Seyfert 1 ULIGs with those of optically selected quasars of comparable bolometric luminosity suggest that the dominant energy source in most of these ULIGs is the same as in optical quasars, namely mass accretion onto a supermassive black hole, rather than a starburst. These results are consistent with recently published ISO, ASCA, and VLBI data. On the other hand, there is no unambiguous optical or near-infrared spectroscopic evidence for an AGN in ULIGs optically classified as H II-region galaxies (∼ 30% of the Department of Astronomy, University of Maryland, College Park, MD 20742; E-mail: [email protected] Visiting observers at the Kitt Peak National Observatory of the National Optical Astronomy Observatories, operated by AURA, Inc., for the National Science Foundation. Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125; E-mail: [email protected] Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822; E-mail: [email protected]