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C.M. Ishida, D.B. Sanders (Institute for Astronomy, University of Hawaii)
Deep V-band imaging and optical color maps from B,V,R and I-band images have been obtained for a complete sample of 56 luminous infrared galaxies (LIGs: LIR = 1011-1012 Lsun) drawn from the IRAS Revised Bright Galaxy Survey. These data have been combined with new and published multiwavelength data from the literature (e.g. CO(1-0), K-band images, optical spectra, VLA continuum images, HI 21cm-line) in order to characterize the morphology and star formation properties of LIGs. We find clear evidence for an increasing fraction of strongly interacting/merger pairs of large, relatively equal mass (~L*), molecular gas-rich spirals with increasing LIR. At LIR < 1011.3 Lsun interactions involving small mass companions dominate, while at higher luminosities interactions involving spiral pairs with mass ratios < 3:1 are most common. There appears to be a weak trend of decreasing nuclear separation with increasing LIR. Fifty percent of LIGs have HII optical spectra, 17% are LINERs, an 34% are Seyferts. There are no obvious trends as a function of LIR. Millimeterwave interferometry coupled with (R-K) aperture photometry suggests large concentrations of molecular gas within the central few kiloparsecs of the merger systems. Comparison of our results with similar existing data for a complete sample of ultraluminous infrared galaxies (LIR > 1012 Lsun) suggests that a substantial fraction of LIGs with LIR > 1011.3 Lsun may eventually pass through a ULIG phase.