Studies of IRAS sources at high galactic latitudes – IV. New redshifts and the spectroscopic properties of IRAS galaxies

We present redshifts, Hα line fluxes and optical continuum fluxes for IRAS galaxies observed while completing the redshift survey, the statistical results of which have been summarized in Paper II in the series. The newly observed objects constitute the majority of the most IR-luminous objects in the flux-limited sample, so that their spectroscopic properties are of independent interest. Nearly all the galaxies show strong emission lines, usually Hα+[N II] and [S II]. Only 6 per cent (12) of the galaxies show high-excitation lines. Four of these are likely type 1 Seyferts and the remaining eight are likely type 2 Seyferts. The high-excitation galaxies tend to be more luminous at 60μm than average, and have warmer IRAS colours. The majority of the IRAS galaxies show low-excitation spectra, similar to H II regions. Where [O I] λ6300 is seen, it is typically about 1/20 the strength of Hα. The more luminous galaxies tend to have larger Hα equivalent widths and warmer IRAS colours. The optical colours from 5500 to 9000 Å reflect a normal underlying old stellar population. Emission line ratios indicate a typical extinction of $A_v\approx4$. The IR excess, based on the ratio of $L_{60\mu m}/L_{\text H\alpha}$ and the typical reddening, is usually larger than seen in Galactic H II regions, but almost always smaller than those of ARP 220 and NGC 6240. We suggest that the large IR excess may be due to the presence of embedded ionizing sources, and dust competition for ionizing photons. Consequently, whatever the underlying energy source, most of the energy emerges in the IR.