Nuclear Water Maser Emission in Centaurus A
  We report the detection of a 22GHz water maser line in the nearest (D~3.8Mpc)
radio galaxy Centaurus A using the Australia Telescope Compact Array (ATCA).
The line is centered at a velocity of ~960kms-1, which is redshifted by about
400kms-1 from the systemic velocity. Such an offset, as well as the width of
~120kms-1, could be consistent with either a nuclear maser arising from an
accretion disk of the central supermassive black hole, or for a jet maser that
is emitted from the material that is shocked near the base of the jet in
Centaurus\,A. The best spatial resolution of our ATCA data constrains the
origin of the maser feature within <3pc from the supermassive black hole. The
maser exhibits a luminosity of ~1Lo, which classifies it as a kilomaser, and
appears to be variable on timescales of months. A kilomaser can also be emitted
by shocked gas in star forming regions. Given the small projected distance from
the core, the large offset from systemic velocity, as well as the smoothness of
the line feature, we conclude that a jet maser line emitted by shocked gas
around the base of the AGN is the most likely explanation. For this scenario we
can infer a minimum density of the radio jet of ~>10cm-3, which indicates
substantial mass entrainment of surrounding gas into the propagating jet
material.
