Water-Vapor Maser Emission from the Seyfert 2 Galaxy IC 2560: Evidence for a Super-Massive Black Hole

We present an H$$_2$$O maser emission study of the Seyfert galaxy IC 2560 based on single-dish and VLBI observations. The maser emission was detected in multiple velocity ranges: one around the systemic velocity, one blue-shifted from the systemic velocity by $$\sim220$$-$$420~$$km$$~$$s$$^{-1}$$, and one red-shifted by $$\sim210$$-$$350~$$km$$~$$s$$^{-1}$$. This was the first detection of high-velocity features in this galaxy. The velocity of the systemic features drifts at a mean rate of $$2.62\pm0.09~$$km$$~$$s$$^{-1}~$$yr$$^{-1}$$. Assuming a compact Keplerian disk at the nucleus, the inner and outer radii of this disk are 0.07 and 0.26 pc, respectively. The central mass confined within the disk inner radius is $$2.8\times10^6~M_{\odot}$$, and its density is at least $$2.1\times10^9~M_{\odot}~\mbox{pc}^{-3}$$. Such a high density strongly suggests the existence of a black hole at the nucleus. The maser spots of the systemic features are distributed over $$\lt 0.3~$$mas (0.04 pc). The continuum source at the same position as the maser source is $$\lt 1.0~$$mas (0.13 pc) in size, and its brightness temperature of $$2.7\times10^{11}~$$K confirms IC 2560 to be an AGN. The 2-10 keV luminosity based on ASCA data is $$1.0\times10^{41}~$$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 $$2\times10^{-5}~M_{\odot}~\mbox{yr}^{-1}$$ in order to explain the X-ray luminosity.