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C.G. Mundell, A.S. Wilson (University of Maryland), J.S. Ulvestad (NRAO), A.L. Roy (NRAO (present address MPIfR))
We present high angular resolution (~2mas) radio continuum observations of five Seyfert galaxies with flat spectrum radio cores, using the VLBA at 8.4GHz. The goal of the project is to determine whether these flat spectrum cores represent thermal emission from the accretion disk, as found previously in NGC 1068, or non-thermal, synchrotron self-absorbed emission, which is believed to be responsible for more powerful flat spectrum nuclear sources in radio galaxies and quasars. In three sources (NGC 2110, NGC 5252, Mkn 926), the nuclear source is detected but unresolved by the VLBA, indicating brightness temperatures in excess of 108 K. We argue that the radio emission is non-thermal and self absorbed in these galaxies. In one galaxy (NGC 2110), the source is extended by ~0.2 pc along the same direction as the 400 pc-scale jet seen with VLA observations, suggesting that the radio emission comes from the base of the jet. In one source (NGC 4388), the flat spectrum nuclear source is undetected by the VLBA. We also present MERLIN and VLA observations of this galaxy and discuss a thermal model for the radio emission.
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