The Megamaser Cosmology Project. X. High-resolution Maps and Mass Constraints for SMBHs

We present high-resolution (submas) Very Long Baseline Interferometry maps of nuclear H₂O megamasers for seven galaxies. In UGC 6093, the well-aligned systemic masers and high-velocity masers originate in an edge-on, flat disk and we determine the mass of the central supermassive black holes (SMBH) to be M _SMBH = 2.58 ×10⁷ M _o (±7%). For J1346+5228, the distribution of masers is consistent with a disk, but the faint high-velocity masers are only marginally detected, and we constrain the mass of the SMBH to be in the range (1.5-2.0) ×10⁷ M _o. The origin of the masers in Mrk 1210 is less clear, as the systemic and high-velocity masers are misaligned and show a disorganized velocity structure. We present one possible model in which the masers originate in a tilted, warped disk, but we do not rule out the possibility of other explanations including outflow masers. In NGC 6926, we detect a set of redshifted masers, clustered within a parsec of each other, and a single blueshifted maser about 4.4 pc away, an offset that would be unusually large for a maser disk system. Nevertheless, if it is a disk system, we estimate the enclosed mass to be M _SMBH < 4.8 ×10⁷ M _o. For NGC 5793, we detect redshifted masers spaced about 1.4 pc from a clustered set of blueshifted features. The orientation of the structure supports a disk scenario as suggested by Hagiwara et al. We estimate the enclosed mass to be M _SMBH < 1.3 ×10⁷ M _o. For NGC 2824 and J0350-0127, the masers may be associated with parsec- or subparsec-scale jets or outflows.