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Abstract
A flux limited sample of 23 optically identified galaxies has been selected from Infrared Astronomical Satellite (IRAS) data that have far-infrared luminosities greater than $\sim$5 $\times$ 10$\sp{10}$L$\sb\odot$, substantially larger than those of normal spiral galaxies. If star formation is the cause of the large luminosities, then strong hydrogen recombination line emission should be present, although substantial extinction is probable. Accordingly, the sample has been observed for Br$\alpha$ (n = 5 $\to$ 4) hydrogen recombination line emission; 15 of the galaxies have had Br$\gamma$ (n = 7 $\to$ 4) line measurements. Sixteen of the sample galaxies were detected in one or both of the lines. Additional infrared photometry and silicate optical depth measurements have been used along with measured line ratios to correct the observed line strengths for the effects of extinction and spatially-extended emission.
Most of the galaxies have line strengths that can be adequately explained by recombination of hydrogen ionized by the continuum radiation from high mass stars. In particular, the ratio of the galaxies' luminosity to the estimated number of ionizing continuum photons is what would be expected from a burst of star formation. The lines are also spectrally unresolved, as expected for a collection of low internal dispersion HII regions taking part in only galactic rotation. Therefore, most of the sample galaxies appear to derive most of their luminosities from star formation.
There are several galaxies that have weak lines and very few ionizing continuum photons for their luminosities, however. The best example is IC 4553 (= Arp 220). The Br$\alpha$ emission in IC 4553 is $\sim$10 times smaller than expected from a starburst and has very broad profile, with a FWHM of $\sim$1200 km s$\sp{-1}$ and possibly even broader wings. NGC 6240 and the known Seyfert galaxies Mkn 231 and NGC 7469 also have relatively weak lines and broad profiles. Several other galaxies have weak lines, but not conclusively broadened profiles (e.g., NGC 4418 and IC 694). These characteristics are contrary to what is expected from a starburst, but are similar to the observed properties of other Seyfert galaxies. Therefore, these objects may have their luminosities generated by an active galactic nucleus.
