Authors: Tien Ho, Dieu Nguyen, Alister W. Graham, Mark Durre, Hai Ngo, Tinh Le, Tuan Le, Elena Gallo, Igor Chilingarian, Fan Zou, and Roberto Soria
Abstract: We present the first stellar-dynamical supermassive black hole mass measurement in the nearby spiral galaxy NGC~5055 (M63) using near-infrared integral-field spectroscopy with Keck/OSIRIS assisted by the laser guide star adaptive optics system. The observations deliver two-dimensional stellar kinematics extracted from the CO bandhead absorption features via PPFX at an angular resolution of 0.045 arcsec. The stellar velocity dispersion rises steeply from ~90 km s-1 at r ~ 1 arcsec to a central peak of ~135 km s-1, kinematically tracing the gravitational influence of a central mass concentration. Modeling these kinematics with an ensemble of four JAM models, spanning cylindrical and spherical velocity-ellipsoid alignments and two independent MGE decompositions of archival HST imaging, yields a black hole mass of MBH=(4.56+1.73-1.82)×106 Msolar. The stellar orbits are consistent with near-isotropy (β ≈ 0), typical of rotating late-type galaxies. NGC 5055 emerges as a remarkable local analogue to the Milky Way in terms of its black hole, nuclear star cluster, and total stellar mass. The surface brightness profile of NGC~5055 shows no evidence for a classical bulge; if confirmed bulgeless, NGC 5055 would join the rare class of spirals with a direct dynamical Mbh determination in the absence of a bulge, a regime currently populated by only a handful of galaxies including NGC~2748 and NGC~4395. This measurement provides a new anchor for black hole scaling relations in late-type galaxies at the low-mass end of the black hole mass spectrum.