1998ApJ...508..627K

This is the second paper in a series discussing the properties of ultraluminous infrared galaxies (ULIGs: L_IR > 10¹² L_☉; H₀ = 75 km.s^–1 Mpc^–1 and q₀ = 0.0) from the 1 Jy sample of Kim. This paper presents the first results of a spectroscopic survey at optical wavelengths of a randomly selected subset of 45 ULIGs from Kim & Sanders. These new data are combined with previous data from Veilleux et al. to determine the spectral properties of luminous infrared galaxies (LIGs) with L_IR ~ 10^10.5-10¹³ L_☉. We find that the fraction of Seyfert galaxies among LIGs increases dramatically above L_IR ~ 10^12.3 L_☉–nearly one-half of the galaxies with L_IR > 10^12.3 L_☉ present Seyfert characteristics. Many of the optical properties of these Seyfert galaxies are consistent with the presence of genuine active galactic nuclei (AGNs) in the cores of these objects. The continuum colors and strengths of the stellar Hβ and Mg I b features in and out of the nuclei of ULIGs indicate that star formation has recently (∼10⁷ yr) taken place in the nuclear and circumnuclear regions of many of these objects. As expected, photoionization by hot stars appears to be the dominant source of ionization in the objects with H II region-like spectra. Evidence is presented that the ionization source in infrared-selected galaxies with nuclear LINER-like spectra (38% of the ULIGs in our sample) is likely to be shocks or of stellar origins rather than AGNs. Shock ionization associated with starburst-driven outflows may also explain the LINER-like emission detected outside the nuclei of some galaxies. No significant differences are found between the mean color excess of ULIGs and that of IRAS galaxies of lower infrared luminosity. However, in contrast to what was found in low-luminosity infrared galaxies, the color excess in the nuclei of ULIGs does not seem to depend on spectral types. The reddening in ULIGs is generally observed to decrease with distance from the nucleus as in their low-luminosity counterparts. In less than 1/5 of the ULIGs in our sample, we observe clear cases of inverted reddening profiles, nearly all of which are optically classified low-ionization nuclear emitting regions (LINERs). These profiles may reflect the actual dust distribution in these objects or may be due to complex optical depth effects.