An Optical Investigation of Powerful Far-Infrared Galaxies.

The Infrared Astronomical Satellite (IRAS) surveyed over 96% of the sky at 12, 25, 60, and 100mu . One of the most interesting results of the mission was the discovery of a class of powerful far-infrared galaxies (FIRG's) having both high far-infrared luminosities (10 ¹⁰-10^ {12} L _odot) and large IR-to-optical flux ratios (10-100). The most extreme examples of these galaxies emit >=q 95% of their total luminosity in the far-infrared. The "prototype" FIRG's (e.g. Arp 220, NGC 3690, NGC 6240, and M82) have been modeled as galaxies undergoing large bursts of star formation, and/or galaxies harbouring active galactic nuclei (AGN). To generate a large sample of FIRG's suitable for a statistical analysis of the ultimate source of their energies, we have applied a mid-far-infrared spectral shape criteria 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 FIRG's. We present spectra and/or images (narrow-band Hα + (NII) and broadband R) of a total of 71 ICSS galaxies. Nearly 80% of the ICSS FIRG's exhibit disturbed continuum morphologies, many of which are strikingly similar to those produced in simulations of colliding disk galaxies. We find spectroscopic evidence, in the form of stellar absorption and emission-lines, continuum colors, and nebular emission-lines, suggesting that the ICSS FIRG's have recently (in the past ~ 10^{rm 7} -10^{rm 8} yr.) undergone episodes of enhanced circumnuclear star formation. Narrow-band imaging reveals that as a class, the ICSS FIRG's possess emission-line nebulae that are both large (mean half light radius ~1.3 Kpc), and luminous (mean total Hα + (NII) luminosity ~10 ^8 L_odot). The ICSS FIRG's have nebular morphologies that range from smooth and structureless to highly complex and chaotic, with some showing evidence for ordered structures (loops, bubbles etc) on scales of a few Kpc to tens of Kpc. Detailed studies of the most nearby FIRG's have revealed that large scale supernovae/massive star-driven winds make a significant contribution to the morphology and kinematics of the ionized gas in these galaxies. We propose that a typical ICSS FIRG be visualized as a "super -giant" HII-region: a galaxy whose interstellar medium is being ionized (on a Kpc to ten Kpc scale) by UV radiation and mass outflows as a direct result of a circumnuclear burst of star formation. This star formation has been triggered by the highly-inelastic collision and subsequent merger of two disk galaxies.