Optical Spectroscopy of theIRAS1 Jy Sample of Ultraluminous Infrared Galaxies

This paper discusses the optical spectroscopic properties of the IRAS 1-Jysample of ultraluminous infrared galaxies (ULIGs). One hundred and eight of the118 1-Jy ULIGs have been observed at dlambda = 8.3 AA resolution over thewavelength range ~4500 A -- 8900 A. These data are combined with large,previously published sets of optical spectroscopic data of lower luminosityinfrared galaxies to look for systematic trends with infrared luminosity overthe luminosity range L_ir ~ 10^{10.5}-10^{13} L_sun. As found in previousstudies, the fraction of Seyfert galaxies among luminous infrared galaxiesincreases abruptly above L_ir ~ 10^{12.3} L_sun --- about 50% of the galaxieswith L_ir > 10^{12.3} L_sun present Seyfert characteristics. Many of theoptical and infrared spectroscopic properties of the Seyfert galaxies areconsistent with the presence of a genuine active galactic nucleus (AGN). About30% of these galaxies are Seyfert 1s with broad-line regions similar to thoseof optical quasars. The percentage of Seyfert 1 ULIGs increases with infraredluminosity, contrary to the predictions of the standard unification model forSeyfert galaxies. Comparisons of the broad-line luminosities of optical andobscured Seyfert 1 ULIGs with those of optically selected quasars of comparablebolometric luminosity suggest that the dominant energy source in most of theseULIGs is the same as in optical quasars, namely mass accretion onto asupermassive black hole, rather than a starburst. These results are consistentwith recently published ISO, ASCA, and VLBI data. (abridged)Comment: Text and 23 figures (45 pages), Tables 1 - 6 (16 pages