The IRAS 1 Jy survey of ultraluminous infrared galaxies

A complete flux-limited sample of 115 ultraluminous infrared galaxies (ULIGs: {dollar}rm Lsb{lcub}it ir{rcub} > 10sp{lcub}12{rcub}Lsb{lcub}odot{rcub}{dollar}; {dollar}Hsb{lcub}o{rcub} = 75 km ssp{lcub}-1{rcub}Mpcsp{lcub}-1{rcub}, qsb{lcub}o{rcub} = 0.5{dollar}) has been identified from the IRAS Faint Source Catalog (FSC). The selection criteria were {dollar}Fsbnu{dollar}(60{dollar}mu{dollar}m) {dollar}ge{dollar} 1 Jy in a region of the sky {dollar}delta > -40spcirc , vert bvert > 30spcirc{dollar}. The maximum luminosity observed is {dollar}Lsb{lcub}ir{rcub} = 10sp{lcub}12.84{rcub} Lsbodot{dollar}, and the maximum redshift is z = 0.27. The luminosity function for ULIGs can be approximated by {dollar}Phipropto Lsbsp{lcub}ir{rcub}{lcub}3.39pm 0.30{rcub} lbrack Mpcsp{lcub}-3{rcub}dexsp{lcub}-1{rcub}rbrack{dollar}. A maximum likelihood test suggests strong evolution for our sample. Assuming density evolution proportional to (1 + z){dollar}sp{lcub}alpha{rcub}{dollar}, we find {dollar}alpha{dollar} = 7.5 {dollar}pm{dollar} 3.1. Examination of the two-point correlation function shows marginal evidence for clustering on a size scale {dollar}rsim 24 hsp{lcub}-1{rcub}Mpc{dollar}. Optical and near-infrared images indicate that almost all of the ULIGs are strongly interacting/merger systems. Analysis of the radial surface brightness profiles of those ULIGs identified as advanced mergers suggests that {dollar}sbsp{lcub}sim{rcub}{lcub}>{rcub}{dollar}30% of these objects have already become ellipticals. Optical spectra show that both the fraction of active galactic nuclei (AGN) in ULIGs and the fraction of Seyfert galaxies among these AGN increases rapidly above log (L{dollar}sb{lcub}rm ir{rcub}{dollar}/L{dollar}sb{lcub}odot{rcub}{dollar}) {dollar}approx{dollar} 12.3. Many of the optical, IRAS, and radio properties of these Seyfert galaxies are consistent with the presence of a genuine AGN in the core of these objects. Photoionization by hot stars appears to be the dominant source of ionization in those objects with H II region-like spectra. We present additional evidence that the ionization source in the nuclei of objects with LINER-like spectra is likely to be shocks or of stellar origin rather than an AGN. The reddening in ULIGs is generally observed to decrease with distance from the nucleus as is observed in their lower-luminosity counterparts. However, in contrast to what was found in low-luminosity IRAS galaxies, the color excess in ULIGs does not seem to depend on optical spectral type. Moreover, objects with inverted reddening profiles appear to be more common among ULIGs than in objects of lower luminosity.