High-luminosity IRAS galaxies. II - Optical spectroscopy, modelling of starburst regions and comparison with structure

Moderate-resolution spectrophotometry was used to obtain various emission-line ratios and emission-line luminosities for a complete sample of predominantly high-luminosity IRAS galaxies. Most of the objects exhibit H II region-like spectra, while about 12 percent are Seyferts or LINERs. The results show the IRAS galaxies to be of lower ionization than an optically selected sample of H II region-like galaxies, possibly due to either high metallicities or to their high dust content. Although the estimated number of O stars present is consistent with the observed emission-line flux, the IR to emission-line luminosity ratio of all the IRAS galaxies is very high. The present observations can be reconciled using a model with two types of regions, type I clouds (with extinctions of about 20) representing very recent star formation, and type II clouds (with extinctions of about 1) representing older starburst and/or general disk star formation.