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 a Seyfert 2, IC 2560. We monitored the velocities of the maser features with the 45-m telescope of the Nobeyama Radio Observatory. Using data form 1995-2006, the velocity drift rate was detected to be (a) over bar = +2.57 +/- 0.04 km s(-1) yr(-1) on the average for 6 systemic features. The Very Long Baseline Array (VLBA) with the Very Large Array (VLA) firstly detected red-shifted and 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 a position angle of PA = -46 degrees, which is almost perpendicular to the galactic disk. Assuming 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) M-circle dot at a distance of D = 26 Mpc, and the mean volume density within the inner radius is 1.3 x 10(9) M-circle dot pc(-3), strongly suggesting a massive black hole at the center. A continuum component was detected at 0.2 pc southwest of the disk center, and is considered to be a jet ejected from the nucleus, with an angle of 70 degrees 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 was estimated to be D = 31(-14)(+12) Mpc from the geometry of the maser disk and the velocity drift rate.