The Iras 1-jy Sample of Ultraluminous Infrared Galaxies: Ii. Optical Spectroscopy

This is the second paper in a series discussing the properties of ultraluminous infrared galaxies (ULIGs: Lir > 10 12 L⊙; Ho = 75 km s −1 Mpc and qo = 0.0) from the 1-Jy sample of Kim (1995). This paper presents the first results of a spectroscopic survey at optical wavelengths of a randomly selected subset of 45 ULIGs from Kim & Sanders (1997). These new data are combined with previous data from Veilleux et al. (1995) to determine the spectral properties of luminous infrared galaxies (LIGs) with Lir ≈ 10 10.5 − 10 L⊙. We find that the fraction of Seyfert galaxies among LIGs increases dramatically above Lir ≈ 10 12.3 L⊙ — nearly half of the galaxies with Lir > 10 12.3 L⊙ present Seyfert characteristics. Many of the optical properties of these Seyfert galaxies are consistent with the presence of a genuine active galactic nucleus (AGN) in the core of these objects. The continuum colors and strengths of the stellar Hβ and Mg Ib features in and out of the nuclei of ULIGs indicate that star formation has recently (∼ 10 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 an AGN. 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 constrast 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 This work was part of a Ph.D. thesis by D.–C. Kim completed in the Department of Physics and Astronomy, University of Hawaii, Honolulu, HI Current address: 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 Department of Astronomy, University of Maryland, College Park, MD 20742; E-mail: [email protected] Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822; [email protected]