THE STARBURST-AGN CONNECTION - II - THE NATURE OF LUMINOUS INFRARED GALAXIES AS REVEALED BY VLBI, VLA, INFRARED, AND OPTICAL OBSERVATIONS

We present here detailed results of an 18 cm VLBI survey of 31 luminou s (L-FIR > 10(11.25) L.), radio-compact (theta less than or similar to 0.'' 25) infrared galaxies (LIGs). High-resolution VLA maps at 15 and 22 GHz are presented for 14 of these galaxies that exhibit compact mi lliarcsecond-scale emission, providing information about radio structu re of LIGs on scales from 0.'' 004 to 1.'' 0. We also present new opti cal spectrophotometric observations. Over half the sample galaxies sho w high brightness temperature radio emission from the VLBI data, with T-b > 10(5) K and structure on scales of 5-150 mas, as previously repo rted in Lonsdale, Smith, & Lonsdale. The median VLBI power for detecte d sources is log P-VLBI = 22.0 (W Hz(-1)), and the mean ratio of VLBI to total 1.6 GHz flux density is [S-VLBI/S-total] = 0.12. Further stru cture is observed on the larger VLA scales. No highly significant (P < 1%) statistical correlations are found between the presence or streng th of the VLBI emission and other observed quantities, including total radio power, radio spectral index, IR luminosity and colors, radio-in frared ratio, molecular gas mass, and optical excitation. Statistical analysis does suggest that the infrared luminosity, molecular gas emis sion, and radio emission on VLA and VLBI scales are physically related . Previous work (Lonsdale, Smith, & Lonsdale) demonstrated that hidden (dust-enshrouded) active galactic nuclei (AGNs) are capable of poweri ng LIGs and giving rise to the observed VLBI- and VLA-scale structures ; here we investigate the complementary question of whether a starburs t can completely explain the observed characteristics, including the h igh brightness temperature radio emission. Simple starburst models sho w that the far-infrared luminosity can be explained by starbursts in a ll cases except Mrk 231, although for some objects the constraints imp osed on the initial mass function are severe. Using our starburst mode ls we model the VLBI data for 11 galaxies with detailed radio structur al information using complexes of radio supernovae. The required super nova rates are nu(sn) similar to 0.1-2 yr(-1), consistent with the rat es derived from the starburst model to explain the observed far-infrar ed luminosities. However in all cases we require complexes of extremel y luminous radio supernovae (RSNs) to explain the high-T-b emission. I n some cases the RSN must have implausibly high radio powers, more tha n an order of magnitude larger than any previously reported RSNs; in o ur view these sources represent AGN radio cores. In most cases an acce ptable fit requires that the RSN be clustered on parsec scales. Furthe rmore, only a few clumps may be active in the radio at a given time. B ased on this analysis we conclude that 7/11 systems can be plausibly e xplained as starbursts. Four galaxies, UGC 2369, Mrk 231, UGC 5101, an d NGC 7469, almost certainly house AGN radio cores. From our modeling, coupled with other recent VLBI and infrared evidence, we conclude tha t Arp 220 is dominated by a massive starburst at radio and infrared wa velengths.