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

We have obtained moderate-resolution spectrophotometry, and here present various emission-line ratios and emission-line luminosities, for a complete sample of (predominantly high-luminosity) IRAS galaxies. Line ratio diagnostic diagrams show most to exhibit H II region-like spectra, although about 12 per cent are Seyferts or LINERs. The fraction of active galaxies does not appear to be a function of IR luminosity. The typical extinction, as derived from Hα/H{BETA} is A_v_ ~1. Comparison of the [O III] H{BETA} line ratios of IRAS galaxies with those of an optically selected sample of H II region-like galaxies shows the IRAS galaxies to be of lower ionization, which may be due to either higher metallicities or their high dust content. The IRAS galaxies show a range of optical colours, with the majority having colours similar to Sc galaxies. Some galaxies, however, have colours indicative of AGN. Comparison with population synthesis models enables us to estimate the number of O stars present, which is consistent with the observed emission-line flux. The IR to emission-line luminosity ratio of all the IRAS galaxies is, however, very high, suggesting a much higher optical depth. We reconcile these observations by presenting a simple model with two types of region: Type I clouds representing very recent star-formation with A_v_ ~20 and Type II clouds with A_v_ ~1, representing older starburst and/or general disc star-formation.