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

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We present a study of H₂O 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 ≈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_ rot=$ 124–168 km s$⁻¹$, having a large velocity dispersion of $ V≈ 31 km s⁻¹. The rotation curve of the disk is sub-Keplerian (V_ rot-0.19 ± 0.04), indicating a massive disk. The disk mass is estimated to be (4.3±0.3) × 10⁷ M and the mass of a putative central black hole is <10⁷ M. The massive disk is gravitationally unstable. A radio continuum source was also detected to be offset from the maser disk by 1.6 pc.