IRAS LRS SPECTROSCOPY OF GALAXIES

IRAS low resolution spectrometer (LRS) data have been sought for 350 galaxies with pointlike IRAS sources having either S(12) or S(25) greater-than-or-equal-to 1.5 Jy. Techniques have been developed for forming the mean of an ensemble of LRS spectra, all of which are only of low signal-to-noise ratio, by quantitative evaluation of the significance of the individual spectra for each object rather than mere acceptance of the "average spectrum" present in the complete LRS database. Galaxies have been selected from the IRAS Bright Galaxy Sample (using only objects unresolved by IRAS), from early IRAS Circulars, from Cataloged Galaxies and Quasars Observed in the IRAS Survey, and from several color-based selections, some unpublished, of galaxies drawn from the Point Source Catalog. The aim was to form the average LRS spectra (lambda-lambda 7.7-22.7-mu-m) for groups of galaxies with distinct optical nuclear properties. Average LRS spectra for several categories of object are presented and interpreted. I find H II region galaxies show the polycyclic aromatic hydrocarbon spectrum of bands in emission; Seyferts of type 2 present a broad emission feature peaking near 16-mu-m; LINERs and galaxies without optical emission lines have LRS spectra that decline with wavelength, whereas Seyferts of type 1 and WR galaxies have red spectra suggestive of nonthermal emission processes.