The nature of flat-spectrum nuclear radio emission in Seyfert galaxies

The power source for nuclear activity in Active Galactic Nuclei (AGNs) is thought to be accretion of material onto a central supermassive blackhole, with infalling material forming an accretion disk which then regulates the fuelling rate. The radii to which these disks extend is not well established but current AGN unification schemes advocate a geometrically thick and clumpy torus or warped thin disk that hides the nucleus when viewed edge-on and accounts for the observed differences between broad-line and narrow-line AGN. High angular resolution (VLBI) radio observations of neutral and ionised gas offer a valuable tool for probing these (often highly optically-obscured) circumnuclear regions on parsec scales in nearby AGNs. We present milliarsecond radio imaging of a small sample of Seyfert galaxies with flat-spectrum radio nuclei, with the goal of testing whether these cores represent thermal emission from the accretion disk, as inferred for NGC 1068 by Gallimore et al., or nonthermal, synchrotron self-absorbed emission, which is believed to be responsible for more powerful, flat-spectrum nuclear sources in radio galaxies and quasars. In most cases, the source has a brightness temperature in excess of 108K and thus represents the synchrotron self absorbed core of a jet. In addition, the search for neutral hydrogen in the obscuring torus will be discussed and new, sub-pc scale observations of HI in NGC 4151 will be presented.