Water-vapor maser emission from the Seyfert 2 galaxy IC 2560: Evidence for a 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 multiple velocity ranges: one around the systemic velocity, one blue-shifted from the systemic velocity by ∼ 220-420 km s^-1, and one red-shifted by ∼ 210-350 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 ± 0.09 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 × 10⁶ M⊙, and its density is at least 2.1 × 10⁹ M⊙ 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 < 0.3 mas (0.04 pc). The continuum source at the same position as the maser source is < 1.0 mas (0.13 pc) in size, and its brightness temperature of 2.7 × 10¹¹ K confirms IC 2560 to be an AGN. The 2-10 keV luminosity based on ASCA data is 1.0 × 10⁴¹ 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 × 10^-5 M⊙ yr^-1 in order to explain the X-ray luminosity.