ar X iv : a st ro - p h / 97 12 08 6 v 1 5 D ec 1 99 7 Water Maser Emission and the Parsec - Scale Jet in NGC 3079

We have conducted VLBI observations at sub-parsec resolution of water maser and radio continuum emission in the nucleus of the nearby active galaxy NGC 3079. The 22 GHz maser emission arises in compact (∼ 0.01 pc at a distance of 16 Mpc) clumps, distributed over ∼ 2 pc along an axis that is approximately aligned with the major axis of the galactic disk. The Doppler velocities of the water maser clumps are consistent with their lying in the inner parsec of a molecular disk with a binding mass ∼ 10M⊙, rotating in the same sense as the edge-on kpc-scale molecular disk observed in CO emission. However, the velocity field has a significant non-rotational component, which may indicate supersonic turbulence in the disk. This distribution is markedly different from that of water masers in NGC 4258, which trace a nearly perfectly Keplerian rotating disk with a binding mass of 3.5 × 10M⊙. The 22 GHz radio continuum emission in NGC 3079 is dominated by a compact (< 0.1 pc) source that is offset 0.5 pc to the west of the brightest maser feature. No bright maser emission is coincident with a detected compact continuum source. This suggests that the large apparent luminosity of the maser is not due to beamed amplification of high brightness temperature continuum emission. At 8 and 5 GHz, we confirm the presence of two compact continuum sources, with a projected separation of 1.5 pc. Both have inverted spectra between 5 and 8 GHz, and steep spectra between 8 and 22 GHz. NGC 3079 may be a nearby, low-luminosity example of the class of compact symmetric gigahertz-peaked spectrum (GPS) radio sources. We detected a third continuum component that lies along the same axis as the other two, strongly suggesting that this galaxy possesses a nuclear jet. Faint maser emission was detected near this axis, which may indicate a second population of masers associated with the jet.