THE STARBURST-AGN CONNECTION - A SENSITIVE VLBI SURVEY OF LUMINOUS IRAS GALAXIES

Results of an 18 cm VLBI survey of 31 luminous (L(fir) > 1-(11.25) L.), radio-compact (theta less-than-or-similar-to 0.''25) infrared galaxies are presented. Over half the sample galaxies show high-brightness temperature emission, with T(b) > 10(5) K and structure on scales of 5-150 mas. The median VLBI core power for detected sources is log P(core) = 22.0 (W Hz-1), and the ratio of core to total 1.6 GHz flux density <S(core)/S(total)> = 0.12. The limits for nondetected sources are similar, consistent with a picture in which most of these galaxies have compact cores at a level of a few percent of the total radio flux density. Characteristics of the extended radio structure, infrared properties, and optical excitation are not good indicators of the detectability of VLBI-scale emission. Structural information and energetic considerations rule out a single supernova interpretation of the compact emission in these galaxies, although we cannot exclude the possibility of several simultaneous extraordinarily luminous radio supernovae within the central few hundred pc3. Our results instead favor the presence of an AGN obscured by starburst-related dust.