2023PASJ...75..722S

We report dual-frequency very long baseline interferometry (VLBI) observations at 22 and 43 GHz toward the nucleus of a nearby radio galaxy NGC 4261. In particular, we present a VLBI image of the 22 GHz H₂O maser line and its location in the circumnuclear region of NGC 4261. H₂O maser emission is marginally detected above the three times the image rms level at channels within a velocity range of approximately 2250-2450 km s^–1, slightly red-shifted with respect to the systemic velocity. H₂O maser emission is located approximately 1 milliarcsecond east of the brightest continuum component at 22 GHz, where the continuum spectrum is optically thick, that is, at the free-free absorbed receding jet by ionized gas. A positional coincidence between H₂O maser emission and an ionized gas disk implies that the H₂O maser emission arises from the near side of the disk, amplifying continuum emission from the background receding jet. If the disk axis is oriented 64° relative to the line of sight, the H₂O maser emission is expected to be at a mean radius of 0.3 pc in the disk. The broad line width of the H₂O maser emission can be attributed to complex kinematics in the immediate vicinity of the supermassive black hole (SMBH), including ongoing gas infall on to the SMBH, turbulence, and outflow. This is analogous to the multi-phase circumnuclear torus model in the nearest radio-loud H₂O megamaser source NGC 1052. An alternative explanation for H₂O maser association is the shock region between the jet and ambient molecular clouds. However, this explanation fails to describe the explicit association of H₂O maser emission only with the free-free absorbed receding jet.