The Black Hole Mass of the Sunflower Galaxy NGC 5055: Direct Measurement via Stellar Kinematics with Keck/OSIRIS Laser Guide Star Adaptive Optics

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 pPXF at an angular resolution of $0.045''$. The stellar velocity dispersion rises steeply from ~90~ km~s⁻¹ at r~1'' 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~ M_. 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.