Water-Vapor Maser Disk at the Nucleus of the Seyfert 2 Galaxy IC 2560
  and its Distance
  We present the results of single-dish and VLBI observations for the
water-vapor masers at the nucleus of the Seyfert 2, IC 2560. We monitored
velocities of the maser features with the 45-m telescope of the Nobeyama Radio
Observatory. Using the data of 1995--2006, the velocity drift rate was detected
to be a = +2.57 +/- 0.04 km/s/yr on the average for 6 systemic features. The
Very Long Baseline Array (VLBA) with the Very Large Array (VLA) firstly
detected a red-shifted and a blue-shifted maser features of IC 2560, in
addition to systemic maser features and a continuum component. We propose a
maser disk in the nuclear region. The systemic and red-shifted features are
emitted from a nearly edge-on disk with the position angle of PA = -46 deg,
which is almost perpendicular to the galactic disk. Assuming the Keplerian
rotation, the radii of the maser disk are r = 0.087--0.335 pc, and the
thickness is 2H <= 0.025 pc. The binding mass is 3.5 x 10^6 Msun at a distance
of D = 26 Mpc, and the mean volume density within the inner radius is 1.3 x
10^9 Msun/pc^3, strongly suggesting a massive black hole at the center. A
continuum component was detected at the 0.2 pc southwest of the disk center,
and considered as a jet ejected from the nucleus, with an angle of 70 deg from
the disk. The blue-shifted maser feature is located on the continuum component,
being interpreted to be a "jet maser". The distance to IC 2560 is estimated to
be D = 31^{+12}_{-14} Mpc from the geometry of the maser disk and the velocity
drift rate.
