Refining the M _BH-V _c scaling relation with H _I rotation curves of water megamaser galaxies

Black-hole-galaxy scaling relations provide information about the coevolution of supermassive black holes and their host galaxies. We compare the black-hole mass-circular-velocity (M _BH-V _c) relation with the black-hole-mass-bulge-stellar-velocity-dispersion (M BH-σ*) relation to see whether the scaling relations can passively emerge from a large number of mergers or require a physical mechanism, such as feedback from an active nucleus. We present Very Large Array H_I observations of five galaxies, including three water megamaser galaxies, to measure the circular velocity. Using 22 galaxies with dynamical M BH measurements and V c measurements extending to large radius, our best-fit M _BH-V _c relation, , yields , , and an intrinsic scatter . The intrinsic scatter may well be higher than 0.51, as we take great care to ascribe conservatively large observational errors. We find comparable scatter in the M BH-σ_* relations, , while pure merging scenarios would likely result in a tighter scaling with the dark halo (as traced by V _c) properties rather than the baryonic (σ_*) properties. Instead, feedback from the active nucleus may act on bulge scales to tighten the M _BH-σ_* relation with respect to the M _BH-V _c relation, as observed.