1999ApJ...522..113V

This paper discusses the optical spectroscopic properties of the IRAS 1 Jy sample (f₆₀>1 Jy) of ultraluminous infrared galaxies (ULIGs: L_IR>10¹² L_☉; H₀=75 km.s^–1.Mpc^–1 and q₀=0). One hundred eight of the 118 1 Jy ULIGs have been observed at Δλ=8.3 Å resolution over the wavelength range ∼4500-8900 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 L_IR~10^10.5-10¹³ L_☉. As found in previous studies, the fraction of Seyfert galaxies among luminous infrared galaxies increases abruptly above L_IR~10^12.3 L_☉– about 50% of the galaxies with L_IR>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 1 galaxies 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 25-60 µm colors) are in fact obscured Seyfert 1 galaxies with broad (≳2000 km.s^–1) 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 AGNs in ULIGs optically classified as H II region galaxies (∼30% of the whole sample) or as LINERs (∼40%). The apparent lack of energetically important AGNs in these objects supports the results from recent mid-infrared spectroscopy with ISO. Photoionization by hot stars from recent starbursts appears to be the dominant source of ionization in the objects with H II region-like spectra, while both hot stars and shocks may contribute to the ionization in ULIGs with LINER-like spectra. The weaker Hβ and Mg I b stellar absorption features, larger Hα emission equivalent widths, and bluer optical continuum colors in objects with higher infrared luminosities suggest that the starbursts took place more recently (≲few times 10⁷ yr) and/or are more important (∼10% of the galaxy mass) in ULIGs than in their lower luminosity counterparts. As found in optically selected starbursts, the emission-line gas in ULIGs is dustier than the stellar populations that are producing the optical continuum. The color excess derived from the Balmer-line ratio does not significantly depend on the infrared luminosity, optical spectral type, or IRAS 25-60 µm color of the luminous infrared galaxies. These results suggest that the optical method used to determine the color excess in infrared galaxies underestimates the amount of dust in the dustier objects.