Global VLBI Observations of HI Absorption toward NGC 3894

Abstract

One of the most important problems in the study of AGN is understanding the detailed geometry, physics, and evolution of the central engines and their environments. The leading models involve an accretion disk and torus around a central black hole. Much of this torus should be comprised of atomic gas, detectable in absorption toward the bright inner radio jets. In the last few years, a number of compact symmetric radio sources have been found to exhibit HI absorption, at or near the systemic velocity, toward the central parsecs. Understanding the kinematics of the HI detected toward the central parsecs of these sources will provide an important test of this model and of unified schemes for AGN. We present results of Global VLBI Network observations at 1.4 GHz toward the active nucleus of the nearby elliptical galaxy NGC 3894 (a.k.a. 1146+596, z=0.01068). The center of activity in this source and the orientation of the jets with respect to our line of sight have been determined using VLBI studies of the proper motions of jet components. The 21 cm atomic hydrogen line is seen in absorption slightly redshifted with respect to the systemic velocity toward the core, jet, and counterjet of this source. The structure of the HI in this source is complicated. We find several distinct components present along the lines of sight to the approaching and receding jets, making interpretation challenging.