Evidence for a 20 Parsec Disk at the Nucleus of Centaurus A

Abstract

We report HST NICMOS observations of the central region of NGC 5128 at 2.2mic and in Paschen Alpha (PA). The continuum images show extended emission typical of an elliptical galaxy and a strong unresolved central source we identify as the nucleus of the galaxy. Its position is consistent with ground-based IR and radio data, and with the peak of reddening found with WF/PC-1. In PA, we detect a prominent elongated structure, centered on the nucleus, extended by ~2" at a position angle of 33deg, and with a major to minor axis ratio of ~2. We interpret this as an inclined, ~40 parsec diameter, thin nuclear disk of ionized gas rather than a jet-gas cloud interaction. We do see several weaker PA features, some of which may be circumnuclear gas clouds shocked by the X-ray/radio jet. The disk is one of the smallest ever observed at the nucleus of an AGN. It is not perpendicular to the jet, but consistent with being oriented along the major axis of the bulge. If it represents the warped outer portion of an accretion disk around a black hole, we conclude that even on the scale of a few parsecs, the disk is dominated by the galaxy gravitational potential and not directly related to the symmetry axis of the AGN.