STARBURST GALAXIES - III - PROPERTIES OF A RADIO-SELECTED SAMPLE

We have analyzed the properties of the 20 most radio-luminous UGC star burst galaxies from Condon, Frayer, & Broderick. Near-infrared images, spectra, and optical rotation curves were presented in Smith et al. I n this paper, we use these data and published radio data to assess the stellar populations, dust contents, ionizing conditions, and dynamics of the starbursts. Certain properties of the star formation occurring in these galaxies differ from those observed locally. The infrared ex cesses (IREs) are lower than and span a narrower range of values than those of Galactic H II regions. The starbursts appear to produce a hig her proportion of ionizing photons than most Galactic H II regions. Co nsequently, the initial mass functions (IMFs) of the starbursts may be more strongly biased toward high-mass star formation. The starbursts may also contain fewer old H II regions than the Milky Way. Furthermor e, the starburst IRE is likely to be influenced by the presence of lar ge reservoirs of gas that absorb a larger fraction of the Lyman contin uum photons. The OB stellar and far-infrared luminosities imply that t he upper mass range of the starburst IMF (M > 10 M-circle dot) is char acterized by a slope of 2.7 +/- 0.2. The starburst IMF thus bears a st rong similarity to that observed in Magellanic OB associations. Optica l line ratios indicate that a range of excitation conditions are prese nt. We conclude that the near-infrared light from many of the starburs ts is dominated by a heavily obscured mixture of emission from evolved red stars and young blue stars with small contributions (approximate to 5 %) from thermal gas and hot dust, under the assumptions that a Ga lactic or SMC extinction law can be applied to these systems and that the true reddening curve follows one of the models currently existing in the literature. In some cases, larger amounts of emission from blue stars or hot dust may be required to explain the observed near-infrar ed colors. The amount of dust emission exceeds that predicted from com parisons with Galactic H II regions. The near-infrared colors of some of the systems may also be influenced by the presence of a low-luminos ity active galactic nucleus (AGN). Emission from blue stars and hot du st, if present, dilutes the observed CO index. The activity in the red der, more luminous systems is strongly peaked. The galaxies hosting th e starbursts exhibit a wide range of morphological and star-forming pr operties. While all of the host galaxies are interacting systems, the nuclear separations of the interacting nuclei range from <1 kpc to> 1 Mpc. The dynamical behavior ranges from relaxed to strongly perturbed. The off-nuclear regions of the galaxies are sites of active star form ation and are characterized by a range of excitation conditions. Spati ally extended LINER emission is consistent with shock excitation produ ced by superwinds or galaxy-galaxy collisions. Violent star formation activity occurs over a larger physical scale in the most active starbu rsts. Systems containing mergers and widely separated nuclei possess s imilar colors and luminosities. The burst properties are most likely r egulated by the internal structures of the interacting galaxies and no t the separations of the interacting galaxies.