VLBI Imaging of Luminous Infrared Galaxies: Active Galactic Nucleus Cores in Markarian 231, UGC 5101, and NGC 7469

We report 18cm VLBI continuum imaging observations at 5 mas resolution forUGC 5101, NGC 7469, and Mrk 231, all part of a sample of Luminous InfraredGalaxies which have been shown to have strong VLBI radio cores. The radiomorphology of these three systems on VLBI scales is AGN-like, with well-definedridgelines and high-brightness yet spatially resolved components. The structureand flux densities of these VLBI components are not consistent with starburstgenerated radio supernovae of the type found in Arp 220. On scales of 100pc theradio continuum in all three objects appears to be dominated by an AGN, not astarburst. Radio emission on larger scales may well originate in a less compactcircumnuclear star-forming region. Confirming and extending VLBI imaging of Mrk 231 by Ulvestad et al. (1999),our continuum image shows a triple structure, with a core and two lobes,classifying it as a Compact Symmetric Object (CS0). If the southern (primary)lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobeadvance speed, $v_a \sim 10^{-4}c$, and an age for the jet/compact source, $<10^6 yr$. We have also imaged the 1667 MHz OH maser emission in Mrk 231, whichis extended on scales of 50--100 milliarcsec (40--80 pc) and probably coincideswith the inner region of the disk which is seen in CO emission and HIabsorption. Among OH megamasers studied at high sensitivity with masresolution, Mrk 231 is unique in the stringent upper limits placed upon theflux density of compact OH structures of the type found in Arp 220 and otherLIGs. It is possible that the circumnuclear environment of Mrk 231 has beensufficiently disrupted by the emergent QSO that the cool, dense cloudsnecessary for such compact masers no longer exist.Comment: 43 pages, 7 figure