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
of approximately one million solar masses, 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 35 million solar masses.
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.
VLBI Map of H2O Maser and 22GHz Radio Continuum Emission in the Inner Parsec
of the Active Galaxy NGC3079
This image shows the distribution of water maser emission with respect to the
22 GHz radio continuum emission in the nucleus of the LINER
galaxy NGC 3079, as observed with the VLBA+VLA phased array in
1995 January. Circles mark the fitted positions of maser features
seen in 0.8 km/s-wide spectral channel images.
Color indicates the LSR Doppler velocity (radio definition)
of the spectral channel,
while the size of the circle
is proportional to log(flux density). The images were produced by
phase-referencing the interferometric visibilities to the spectral
channel containing the maser peak, at the origin of the map. The emission
within 0.2 pc of the maser peak (boxed region) is shown in an expanded
view to the left. Contours show the 22 GHz radio continuum emission,
also phase-referenced to the maser peak. The synthesized beam
has a size of 1.2 x 1.0 mas with a position angle of 5 degrees.
Contours are in units of 3 x (sigma=0.22 mJy/beam).
