VLBA Imaging of Seyfert Galaxy Nuclei near their Central Black Holes

Jim Ulvestad

The Very Long Baseline Array (VLBA) has been used to image the cores of a significant number of Seyfert galaxies and Low Luminosity Active Galactic Nuclei (LLAGNs) at milliarcsecond resolution. For most of the targets, the imaging resolution is better than 0.1 pc, typically 10,000 to 100,000 times the Schwarzschild radii of their central black holes; for a few objects, resolution of 1000 Schwarzschild radii is possible. In most galaxies with jets that have been imaged at multiple epochs, the apparent speeds of jet components are less than 0.1c to 0.2c, quite different from the relativistic jets seen in radio galaxies and quasars; a recent sensitive image of the classical Seyfert NGC 4151 reveals an upper limit of 15,000 km/s (0.05c) to its apparent jet speed. A number of LLAGNs have slightly flat or inverted radio spectra, and all these remain unresolved down to the smallest scales imaged, typically a few hundred microarcseconds. Although these objects superficially resemble accretion flows with low radiative efficiency, the radio/X-ray ratio is far too high for most simple models. Instead, it is likely that compact radio jets on scales of less than 1000 Schwarzschild radii, smaller than can be imaged by the VLBA, dominate their radio emission.