The Nuclear Region of NGC 1068: High-Accuracy Alignment of the Optical and Radio Emission

Abstract

We present the results of a campaign to obtain an accurate registration of Hubble Space Telescope (HST) and MERLIN images of NGC 1068, the prototypical Seyfert 2 galaxy. The final registration is accurate to 100 mas.

A strong anticorrelation between radio and optical emission is revealed; in particular, the radio jet lies on a region of relatively low optical emission and is surrounded by line-emitting clouds. A major exception is the brightest narrow-line region (NLR) knot, cloud B, which lies, at least in projection, along the radio jet. These results can be understood as due to the interaction between the jet and the surrounding medium: in this scenario, the outflowing plasma is sweeping and heating the interstellar gas. As a result of this interaction and compression, the line emission is highly enhanced along the edges of the radio jet. It therefore appears that the morphology of the NLR of NGC 1068 is dominated by the presence of a radio outflow, as already revealed by HST observations of several Seyfert galaxies with extended radio emission.

The optical peak seen in the HST images is located at α = 02^h42^m40.711s, δ = -00°00'47.81" (J2000, FK5), with an error of 80 mas. The hidden nucleus, as determined by HST imaging polarimetry, falls at α = 02^h42^m40.710s, δ = -00°00'48.11" (J2000, FK5). It is offset toward the south (i.e., along the radio axis) with respect to the two southernmost radio components and, in particular, by 170 mas from the inverted spectrum radio component, S1. This does not rule out that S1 is indeed the radio core but suggests that the nucleus of NGC 1068 might be radio silent, or its emission absorbed at radio wavelengths also.

There are several contributions to the final accuracy of the optical-to-radio registration. A significant decrease of the astrometric error budget has to wait for substantial improvements, like those expected from the radio-optical link in the forthcoming HIPPARCOS catalog.

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.