A statistical study of the spectra of very luminous IRAS galaxies - II. Spectral and environmental analysis

Spectroscopic observations of a sample of 73 very luminous IRAS galaxies (log(L-IR/L-circle dot) greater than or equal to 11.5 for H-0 = 50 km s(-1) Mpc(-1), q(0) = 0.5) from the 2 Jy redshift survey catalogue were carried out using the 2.16 m telescope at the Beijing Astronomical Observatory. The observational data, including the optical images (extracted from Digital Sky Survey) and spectra for these galaxies, are presented in Paper I (Wu et al. 1998). In this paper, we give the spectral and morphological classifications for these very luminous IRAS galaxies (VLIRGs). We show that about 60% of VLIRGs exhibit AGN-like spectra (Seyfert 1s, Seyfert 2s, LINER-like galaxies). This fraction goes up to 82% for the ultraluminous IRAS galaxies (ULIRGs) subsample (Log(L-IR/L-circle dot) greater than or equal to 12.0). 56% of the VLIRGs show strong interaction or merging signatures; this fraction rises to 91% for the ULIRGs. These statistical results strongly suggest that interaction triggers nuclear activities and enhances the infrared luminosity. We find that LINER and a mixture type which have optical properties of both HII galaxies and LINERs could be at the transition stage from infrared luminous HII galaxies to AGNs; their main energy production is from starbursts as well as AGNs. Both infrared luminosities and Her equivalent widths increase dramatically as nuclear separations between VLIRGs and their nearest neighbors decrease. There is little doubt that strong starbursts happen in the nuclei of VLIRGs. Assuming class 0 as advanced merger, are construct a simple merger sequence, from morphological classes 1 to 4 (with near or far companions), to class 5 and 6 (interacting pairs and mergers) and then to class 0 (isolated galaxies). Along this sequence, VLIRGs evolve from HII galaxies to AGNs.