Keplerian molecular gas disk and black hole mass of NGC 4751

International audience ; Aims. We aim to measure the mass of the supermassive black hole (SMBH) in the S0 galaxy NGC 4751 using CO J:3−2 emission from the 5″-scale nuclear rotating molecular disk.Methods. We imaged the kpc-scale molecular gas disk in NGC 4751 at 0.″22 (∼28 pc) spatial and 28 km s−1 spectral resolution in the CO J:3−2 emission line and neighboring continuum, with the Atacama Large Millimeter Array (ALMA). We used Hubble Space Telescope (HST) imaging and stellar and ionized gas kinematics at 100 pc to kpc-scales, derived from integral field spectroscopy, to determine the galaxy morphology and the circular velocity attributed to the stellar potential. We used the Markov chain Monte Carlo (MCMC) algorithm in the KINematic Molecular Simulation (KinMS) package to obtain the model parameters that best fit the observed molecular gas kinematics in the ALMA datacube.Results. Strong CO emission was detected over radii of ∼0.″2 to 5″, with isolated CO clumps detected out to 7″. The molecular disk kinematics is axisymmetric and rotation-dominated, with radial velocities between 400 km s−1 and 660 km s−1, a kinematic major axis position angle (PA) of 355°, and an inclination close to 78°. The intrinsic velocity dispersion is ∼16 km s−1, and there is no evidence for significant non-rotational kinematics. The kinematic center of the disk coincides with the compact nuclear 345 GHz source. The SMBH sphere of influence (SOI) is well resolved along all position angles. The (rotation) velocity curve due to the stellar potential (Vradialmax ∼ 430 km s−1) is determined by fitting the luminosity profile of NGC 4751 in an (H-band) image from the Wide Field Camera 3 (WFC3) aboard HST, and constraining the mass-to-light ratio (M/L) at this waveband using the molecular- and ionized-gas kinematics at radii ≳4″, outside the SMBH SOI. Several KinMS fits, all using a distance (D) of 26.3 Mpc, but with variations in other input quantities, resulted in SMBH masses of 3.22 − 4.33 × 109 M⊙ and M/L values of 1.1−2.3 in the F160W ...