VLBI spectral absorption in AGN

Abstract

We present recent VLBI observations of HI and CO gas in AGN, emphasising the variety of phenomena probed by this technique. Broad (>100 km s −1) HI absorption in ellipticals appears to be commonly associated with circumnuclear gas in a TORUS (Thick Obscuration Required by Unified Schemes) at radii of 50–500 pc from the central engine. Examples include Cygnus A and the CSO 4C31.04. In both of these sources recent HST imaging appears to confirm the presence of a TORUS. However, not all broad absorption HI is TORUS related. In 3C 236, for instance, it seems to be due to a jet-cloud interaction. Narrow <100 km s −1 HI absorption lines also appear to be caused by a variety of different phenomena. For instance, VLBI observations show that the narrow, rapidly in-falling HI absorption gas in 4C31.04 is probably associated with High Velocity Clouds similar to those in-falling into our own Galaxy. In contrast, in 1413+135, the narrow HI and molecular absorption is probably due to a foreground molecular cloud in the host's galactic disk which fortuitously lies along the line of sight to the radio nucleus.