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 galaxi es (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 Astronomica l Observatory. The observational data, including the optical images (e xtracted from Digital Sky Survey) and spectra for these galaxies, are presented in Paper I (Wu et al. 1998). In this paper, we give the spec tral and morphological classifications for these very luminous IRAS ga laxies (VLIRGs). We show that about 60% of VLIRGs exhibit AGN-like spe ctra (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 in teraction triggers nuclear activities and enhances the infrared lumino sity. We find that LINER and a mixture type which have optical propert ies of both HII galaxies and LINERs could be at the transition stage f rom infrared luminous HII galaxies to AGNs; their main energy producti on is from starbursts as well as AGNs. Both infrared luminosities and Her equivalent widths increase dramatically as nuclear separations bet ween VLIRGs and their nearest neighbors decrease. There is little doub t that strong starbursts happen in the nuclei of VLIRGs. Assuming clas s 0 as advanced merger, are construct a simple merger sequence, from m orphological classes 1 to 4 (with near or far companions), to class 5 and 6 (interacting pairs and mergers) and then to class 0 (isolated ga laxies). Along this sequence, VLIRGs evolve from HII galaxies to AGNs.