Detection of a Sub-Keplerian Water Maser Disk at the Active Galactic Nucleus of the Galaxy IC 1481

We present a study of H$_{2}$O maser emission from the LINER, IC 1481, based on single-dish and highly sensitive VLBI observations. The Nobeyama 45-m telescope detected new maser features near the systemic velocity and blue-shifted by 124–205 km s$^{-1}$. In addition, the known features red-shifted by $\approx $97–155 km s$^{-1}$were monitored. VLBI observations with the High Sensitivity Array (HSA) revealed that the maser features are distributed along a line, with a velocity gradient, strongly suggesting a nearly edge-on molecular gas disk. The radius and thickness of the disk are $r$$=$2.8–14.0 pc and 2$H$$=$1.5–4.2 pc, respectively, which are the largest among the maser disks detected in active galactic nuclei. The disk is rotating at $V_{{\rm rot}}$$=$124–168 km s$^{-1}$, having a large velocity dispersion of $\triangle V$$\approx $31 km s$^{-1}$. The rotation curve of the disk is sub-Keplerian ($V_{{\rm rot}}$$\propto$$r^{-0.19 \pm 0.04}$), indicating a massive disk. The disk mass is estimated to be (4.3$ \pm $0.3) $\times$10$^{7} M_{\odot }$and the mass of a putative central black hole is $\lt $10$^{7} M_{\odot }$. The massive disk is gravitationally unstable. A radio continuum source was also detected to be offset from the maser disk by 1.6 pc.