A large sample of southern IRAS galaxies: spectral classes and superclustering

We present the results of an extensive spectroscopic survey of IRAS galaxies, in three Southern-Hemisphere fields totaling about 0.5 sr. Spectroscopic observations have been made of 527 galaxies which are candidates for 474 IRAS sources. Together with published data, there are now measured redshifts for almost 850 of the 1178 selected IRAS galaxies in these fields. This measured sample includes nearly all galaxies at high redshifts, and hence high infrared luminosities, within these fields. Classification of more than 350 of these galaxies using emission-line ratios and line widths shows that about 90 percent have starburst characteristics. Although the fraction having Seyfert characteristics increases with luminosity, there remains a significant number of starburst galaxies even at the highest luminosities. Two distinct groups of starburst galaxies in the sample display forbidden [O III] line intensities which significantly exceed those observed in optically selected starburst galaxies. One of these groups, with strong [N II] lines, may be related to liners and/or Seyfert galaxies. The other, with weak [N II] emission, displays unusually narrow lines; we interpret this group in terms of the youngest phase of starburst activity. We derive a luminosity function for IRAS galaxies very similar to one previously published for a different part of the sky, but find a small departure from a simple power law due to an overabundance of galaxies around redshift 0.12. We identify three new large-scale concentrations of galaxies, and suggest that one may be the largest connected structure yet found in the Universe.