The Sub-Parsec Structure of Accretion Disks as Revealed by VLBI Imaging of Free-Free Absorption

The physical conditions in the inner parsec of accretion disks believed to orbit the central black holes in active galactic nuclei can be probed by imaging the absorption of background radio emission by ionized gas in the disk. High angular resolution radio observations of several nearby galaxies at multiple frequencies have revealed evidence for free-free absorption by disks or tori of ionized gas. The depth and angular width of the absorption increases with decreasing frequency. The longest possible baselines are needed to provide adequate angular resolution at low frequencies where the effects of free-free absorption are most evident. Recent results from VSOP as well as ground-based VLBI observations of the nearby galaxy NGC 4261 illustrate the critical importance of high angular resolution at frequencies below 10 GHz.