Water-vapor maser emission from the Seyfert 2 galaxy IC 2560: Evidence fora super-massive black hole

We present an H2O maser emission study of the Seyfert galaxy IC 2560 based on single-dish and VLBI observations. The maser emission was detected in mu ltiple velocity; ranges: one around the systemic velocity, one blue-shifted from the systemic velocity by similar to 220-420 km s(-1), and one rid-shi fted by similar to 210-350 km s(-1). This was the first detection of high-v elocity features in this galaxy. The velocity of the systemic features drif ts at a mean rate of 2.62 +/- 0.09 km s(-1) yr(-1), Assuming a compact Kepl erian disk at the nucleus, the inner and outer radii of this disk an 0.07 a nd 0.26 pc, respectively. The central mass confined within the disk inner r adius is 2.8 x 10(6) M-circle dot and its density is at least 2.1 x 10(9) M -circle dot pc(-3). Such a high density strongly suggests the existence of a black hole at the nucleus. The maser spots of the systemic features are d istributed over < 0.3 mas (0.04 pc). The continuum source at the same posit ion as the maser source is < 1.0 mas (0.13 pc) in size, and its brightness temperature of 2.7 x 10(11) It confirms IC 2560 to be an AGN. The 2-10 keV luminosity based on ASCA data is 1.0 x 10(41) erg s(-1),which makes IC 2560 one of the low-luminosity AGNs. Adopting the standard accretion model, the mass-accretion rate needs to be 2 x 10(-5) M-circle dot yr(-1) in order to explain the X-ray luminosity.