High-Resolution Imaging of Water Maser Emission in the active galaxies
  NGC 6240 and M51
  We present the results of observations of 22GHz H2O maser emission in NGC
6240 and M51 made with the Karl G. Jansky Very Large Array. Two major H2O maser
features and several minor features are detected toward the southern nucleus of
NGC 6240. These features are redshifted by about 300 km/s from the galaxy's
systemic velocity and remain unresolved at the synthesized beam size. A
combination of our two-epoch observations and published data reveals an
apparent correlation between the strength of the maser and the 22GHz radio
continuum emission, implying that the maser excitation relates to the activity
of an active galactic nucleus in the southern nucleus rather than star-forming
activity. The star-forming galaxy M51 hosts H2O maser emission in the center of
the galaxy; however, the origin of the maser has been an open question. We
report the first detection of 22GHz nuclear radio continuum emission in M51.
The continuum emission is co-located with the maser position, which indicates
that the maser arises from active galactic nucleus-activity and not from
star-forming activity in the galaxy.
