Parsec-scale Imaging of the Radio-bubble Seyfert galaxy NGC 6764
  We have observed the composite active galactic nucleus (AGN)-starburst galaxy
NGC 6764 with the Very Long Baseline Array at 1.6 and 4.9 GHz. These
observations have detected a "core-jet" structure and a possible weak
counterjet component at 1.6 GHz. The upper limits to the core and jet (1.6-4.9
GHz) spectral index are 0.6 and 0.3, respectively. Taken together with the high
brightness temperature of ~10^7 K for the core region, the radio emission
appears to be coming from a synchrotron jet. At a position angle of 25 degrees,
the parsec-scale jet seems to be pointing closely toward the western edge of
the southern kpc-scale bubble in NGC 6764. A real connection between the
parsec- and sub-kpc-scale emission would not only suggest the presence of a
curved jet, but also a close link between the AGN jet and the radio bubbles in
NGC 6764. We demonstrate that a precessing jet model can explain the radio
morphology from parsec- to sub-kpc scales, and the model best-fit parameters of
jet speed and orientation are fully consistent with the observed
jet-to-counterjet surface brightness ratio. The jet however appears to be
disrupted on scales of hundreds of parsecs, possibly due to interaction with
and entrainment of the interstellar medium gas, which subsequently leads to the
formation of bubbles. The jet energetics in NGC 6764 suggest that it would take
12-21 Myr to inflate the (southern) bubble. This timescale corresponds roughly
to the starburst episode that took place in NGC 6764 about 15-50 Myr ago, and
could be indicative of a close connection between jet formation and the
starburst activity in this galaxy.
