High-sensitivity VLBI Observations of the Water Masers in the Seyfert Galaxy NGC 1068

Abstract: We present observational results of water vapor maser emission with our high-sensitivity 22 GHz VLBI imaging of the Seyfert galaxy NGC 1068. In this galaxy, there are the following four nuclear radio sources; NE, C, S1, and S2. Among them, the S1 component has been identified as the nucleus while the C component has been considered as attributed to the radio jet. In our VLBI observation, we find the following two types of the water maser emission at the S1 component. One is the linearly aligned component that is considered as an edge-on disk with the inner radius of 0.62 pc. The dynamical mass enclosed within the inner radius was estimated to be $1.5\times10^7 M_{\odot}$ by assuming the circular Keplerian motion. Note, however, that the best fit rotation curve shows a sub-Keplerian rotation ($v\propto r^{-0.24\pm0.10}$). The other is the water maser emission distributed around the rotating disk component up to 1.5 pc from the S1 component, suggesting the bipolar outflow from the S1 component. Further, we detected the water maser emission in the C component for the first time with VLBI, and discovered a ring-like distribution of the water maser emission. It is known that a molecular cloud is associated with the C component (both HCN and HCO$^+$ emission lines are detected by ALMA). Therefore, the ring-like maser emission can be explained by the jet collision to the molecular cloud. However, if these ring-like water masing clouds constitute a rotating ring around the C component, it is likely that the C component also has a supermassive black hole with the mass of $\sim 10^6 M_{\odot}$ that could be supplied from a past minor merger of a nucleated satellite galaxy.