Water-Vapor Maser Disk at the Nucleus of the Seyfert 2 Galaxy IC 2560 and its Distance

TL;DR

This study uses water-vapor maser observations to map the nuclear disk of IC 2560, measuring its structure, black hole mass, and distance, providing insights into the galaxy's central dynamics and geometry.

Contribution

First VLBA and VLA detection of multiple maser features in IC 2560, revealing a nearly edge-on disk, black hole mass, and a new distance estimate based on maser geometry.

Findings

Detected velocity drift rate of +2.57 km/s/yr for systemic features.

Mapped the maser disk with radii 0.087--0.335 pc and mass 3.5 million solar masses.

Estimated galaxy distance as 31 Mpc from maser geometry.

Abstract

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.