Parsec-Scale Imaging of HI Absorption in 1946+708

Abstract

In the last several years, a number of compact extragalactic radio sources have been found to exhibit neutral hydrogen absorption at or near the systemic velocities of their host galaxies. Models proposed to explain this phenomenon involve a circumnuclear torus of gas and dust. The orientation of this structure determines whether or not the central engine appears obscured. Understanding the spatial distribution and kinematics of the HI detected toward the central parsecs of these sources provides an important test of this model and of unified schemes for AGN. We present results of Global VLBI Network observations of the redshifted 21 cm HI line toward the Compact Symmetric Object 1946+708 (z=0.101). This source is of particular interest because it exhibits bi-directional motion measurable on timescales of a few years. The resulting unique information about the geometry of the continuum source greatly assists in the interpretation of the HI distribution. We find significant structure in the gas on parsec scales. The peak column density of the HI occurs near the center of activity of the source, as does the highest velocity dispersion (FWHM ~ 350 to 400 km/s). The distribution of gas in 1946+708 is strongly suggestive of a circumnuclear torus of atomic material with one or more additional compact clumps of gas along the line of sight to the approaching jet.