Abstract
The authors discuss their program of long-slit spectroscopy and narrowband (Hα) imaging of a sample of powerful (≡10<SUP>12</SUP>L_sun;) far-infrared galaxies (FIRGs). The sample was selected so that the mid/far infrared spectral energy distributions resembled those of the prototypes Arp 220 and NGC 6240 ("tepid" FIRGs). The principal new observational results are as follows: (1) As a class, the emission-line spectra of tepid FIRGs are most similar to LINERs, but about half are intermediate between LINERs and low-excitation H II regions. The sample as a whole bears a strong spectroscopic resemblance to the well-known M82 filaments. (2) Huge (10<SUP>4</SUP>- 10<SUP>5</SUP>pc) and morphologically spectacular (multiple filaments, loops, or bubbles) emission-line nebulae resembling the M82 filament system are associated with both NGC 6240 and Arp 220. The authors propose that the model considered by Chevalier and Clegg (1985) and McCarthy, Heckman, and van Breugel (1987) for M82 be generalized to encompass powerful far-infrared galaxies as a class: a starburst-driven bipolar superwind escapes along the rotation axis of a circumnuclear disk and then sweeps up and shock heats gaseous "tidal debris" around the galaxy, producing optical line emission and possibly X rays.
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