An optical investigation of powerful far-infrared galaxies

The Infrared Astronomical Satellite (IRAS) surveyed over 96 percent of the sky at 12, 25, 60, and 100 microns. One of the most interesting results of the mission was the discovery of a class of powerful far-infrared galaxies (FIRGs) having both high far-infrared luminosities and large IR-to-optical flux ratios. The prototype FIRG's were modeled as galaxies undergoing large bursts of star formation, and/or galaxies harboring active galactic nuclei (AGN). To generate a large sample of GIRGs suitable for a statistical analysis of the ultimate source of their energies, a mid-far-infrared spectral shape criteria was applied to the IRAS database. This has resulted in the creation of an Infrared Color-Selected Sample of galaxies (ICSS galaxies), all having infrared spectral energy distributions similar to the prototype FIRGs. Spectra and/or images of a total of 71 ICSS galaxies are presented. Nearly 80 percent of the ICSS FIRGs exhibit disturbed continuum morphologies, many of which are strikingly similar to those produced in simulations of colliding disk galaxies. Spectroscopic evidence was found, in the form of stellar absorption and emission-lines, continuum colors, and nebular emission-lines, suggesting that the ICSS FIRG's have recently undergone episodes of enhanced circumnuclear star formation