The Starburst – AGN Connection II: The Nature of Luminous Far Infrared Galaxies as Revealed by VLBI, Radio, Infrared and Optical Observations

We present here detailed results of an 18 cm VLBI survey of 31 luminous (LFIR > 101 1“25LO), radio-compact (0 ~O!’25) infrared galaxies (LFIRGs). High-resolution VLA maps at 15 and 22GHz are presented for 14 of these galaxies which exhibit compact milliarcsecond scale emission, providing information about radio structure of LFIRGs on scales from O !’004 to 1 !’0. We also present new optical spectrophotometric observations. Over half the sample galaxies show high-brightness temperature radio emission from the VLBI data, with Tb > 10K and structure on scalas of 5 – 150 mas, as previously reported in Lonsdale, Smith and Lonsdale (1993). The median VLBI power for detected [email protected] is log PV[& = 22.0( WHZ-1), and the mean ratio of VLBI to total 1.6 GHz flux density < SV\b;/Stot.l >= 0.12. Further structure is observed on the larger VLA scales. No highly-significant (P < 1%) statistical correlations are found between the presence or strength of the VLBI emission and other observed quantities, including total radio power, radio spectral index, FIR luminosity and colors, radio-infrared ratio, molecular gas mass, and optical excitation. Statistical analysis does suggest that the infrared luminosity, molecular gas emission and radio emission on VLA and VLBI scales are physically related. Previous work (Lonsdale, Smith and Imnsdale 1995) demonstrated that hidden (dust-enshrouded) AGN are capcible of powering LFIRGs and giving rise to the observed VLB1 and VLA scale structures; here we investigate the complementary question of whether a starburst can completely explain the observed characteristics, including the high brightness temperature radio emission. Simple Starburst models show that the far-infrared luminosity can be explained by starbursts in all cases except Mkn 231, although for some objects the constraints imposed on the initial mass function are severe. Using our Starburst models we model the VLBI data for 11 galaxies with detailed radio structural information using complexes of radio supernovae. The required supernova rates are u~n w 0.1-2 yr-l, consistent with the rates derived from the Starburst