VLBI Observations of Symmetric Parsec-Scale Twin Jets in the Narrow-Angle-Tail Radio Galaxy NGC 1265 (3C 83.1B)

Abstract

We present global VLBI observations of the nucleus of the narrow-angle-tail radio galaxy NGC 1265. We report the discovery of symmetric parsec-scale twin jets which are well aligned with the kiloparsec-scale jets. We also present an archival Hubble Space Telescope Wide Field Planetary Camera 2 (WFPC2) image of the galaxy, showing an apparently almost edge-on dust disk within 10 deg of perpendicular to the jet axis. The jets in NGC 1265 are consistent with either (1) relativistic motion near the plane of the sky, (2) nonrelativistic motion, or (3) a ``coaxial'' structure with a nonrelativistic sheath surrounding a relativistic spine. A study of the properties of the nine large-scale sources known so far with twin parsec-scale jets suggest that these sources are more likely to be nonrelativistic or only mildly relativistic. We also suggest that at least some sources with symmetric parsec-scale jets have been decelerated on the parsec scale, possibly through interaction with ambient gas or the shock at the sonic radius of a cooling flow. Other sources may be reborn compact symmetric object-type sources propagating out of the galaxy's nuclear interstellar medium. Further proper-motion studies of this class of source are needed.