THE OPTICAL EMISSION-LINE NEBULAE OF POWERFUL FAR-INFRARED GALAXIES

We present and analyze narrow-band (Hα + [N II]) emission-line images of a sample of 30 powerful far-infrared galaxies (FIRGs) chosen to have far-infrared spectral energy distributions similar to the prototype FIRGs Arp 220, NGC 3690, NGC 6240, and M82. We also present similar images of 10 other powerful far-infrared galaxies not chosen with regard to IR spectral shape. The emission-line nebulae of these IR color-selected (ICSS) FIRGs as a class are impressively large, with a mean half-light radius, re ∼ 1.3 kpc, and mean maximum diameter, D ∼ 16 kpc. They have morphologies that range from smooth and structureless to highly complex and chaotic, with some showing evidence for ordered structure on scales of a few kpc to tens of kpc. Only ∼25% of the nearby (z ≤ 0.10) FIRGs for which we have adequate spatial resolution display morphologies suggesting that large H II regions contribute significantly to their appearance in Hα + [N II]. Of the seven ICSS FIRGs that are apparently highly flattened in R (major-to-minor axis ratio ≥2.0), six have Hα + [N II] isophotes that may be indicating emission-line gas which is extended out of the plane of the galaxy. This type of structure is also seen in three edge-on galaxies that are not members of the IR color-selected sample. The mean total Hα + [N II] luminosity (uncorrected for internal extinction) of the ICSS FIRG nebulae is ∼5 x 1041 ergs s-1, which is a factor of 2-3 greater than that found for individual members of pairs of optically selected interacting galaxies (Bushouse, Lamb, and Werner), and a factor of ∼5 greater than that of isolated spiral galaxies (Kennicutt and Kent). The eight sample galaxies with available Brα/Hα flux ratios have typical extinctions of ∼3 mag at Hα. This extinction then implies an intrinsic Hα + [N II] luminosity of ∼8 x 1042 ergs s-1 for the ICSS FIRGs, which is a factor of ∼30 greater than the average extinction corrected Hα + [N II] luminosity of isolated spiral galaxies of Kennicutt and Kent. The ICSS FIRGs are clearly extraordinary objects in terms of their optical line emission. The good correlation of both Hα + [N II] and far-infrared luminosity with absolute magnitude, along with previous spectroscopic evidence that the optical continuum is produced predominantly by young stars (Armus, Heckman, and Miley), suggests that the emission-line and far-infrared luminosity are also powered by young stars. Using the models of Hunter et al. and Hunter and Gallagher along with the mean far-infrared and extinction corrected Hα luminosity of the FIRGs yields an average total star formation rate of ∼70 M⊙ yr-1. We propose that a typical ICSS FIRG be visualized as a "supergiant" H II region: a galaxy whose interstellar medium is being ionized (on scale of 1-10 kpc) by UV radiation and mass outflows as a direct result of a circumnuclear burst of star formation. While relatively few (≤ 10%) of the ICSS FIRGs are observed to be presently interacting with a conspicuous companion galaxy, the great majority (∼80%) are strongly distorted in continuum morphology. This suggests that our IR selection criteria are preferentially selecting galaxies that have recently undergone a merger.