STScI Preprint #1245

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

Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555 and by STScI grant GO-3594.01-91A

We report HST NICMOS observations of the central region of NGC 5128 at 2.2µm and in 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  33^o, and with Pa 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 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.

Status:

Appeared in: The Astrophysical Journal, 499:L143-L147, 1998

Affiliations:

¹)Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
²) Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, ITALY
³) Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 5, 50125 Firenze, ITALY
⁴) Associated with Astrophysics Division, Space Science Dept., ESA
⁵) On leave from Nuffield Radio Astronomy Laboratory, Jodrell Bank, Univ. of Manchester, UK
⁶)Osservatorio Astronomico di Torino, Strada Osservatorio 20, I-10025 Pino Torinese, ITALY
⁷)Division of Physics and Astronomy, Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield, Herts AL10 9AB, UK
⁸)Isaac Newton Group, Sea Level Office, Apartado de Correos, 321, 38780 Santa Cruz de La Palma, Islas Canarias, SPAIN