2025MNRAS.542.2039D

We present high angular resolution (0.19 arcsec or $\approx 24$ pc) Atacama Large Millimeter/submillimeter Array observations of the $^{12}$CO(3-2) line emission of the galaxy NGC 4751. The data provide evidence for the presence of a central supermassive black hole (SMBH). Assuming a constant mass-to-light ratio ($M/L$), we infer a SMBH mass $M_\text{BH}=3.43^{+0.45}_{-0.44}[\text{stat},\, 3\sigma ]^{+0.22}_{-0.64}[\text{sys}]\times 10^9$ M$_\odot$ and a F160W filter stellar $M/L_\text{F160W}=2.68\pm 0.11[\text{stat},\, 3\sigma ]^{+0.10}_{-0.80}[\text{sys}]$ M$_\odot$/L$_{\odot ,\text{F160W}}$, where the first uncertainties are statistical and the second systematic. Assuming a linearly spatially varying $M/L$, we infer $M_\text{BH}=2.79^{+0.75}_{-0.57}[\text{stat},\, 3\sigma ]^{+0.75}_{-0.45}[\text{syst}]\times 10^9$ M$_\odot$ and $\left(M/L_\text{F160W}\right)/\left(\text{M}_\odot /\text{L}_{\odot ,\text{F160W}}\right)=3.07^{+0.27}_{-0.35}[\text{stat},\, 3\sigma ]^{+0.08}_{-1.14}[\text{sys}]-0.09^{+0.08}_{-0.06}[\text{stat},\, 3\sigma ]^{+0.08}_{-0.01}[\text{sys}]\, \left(R/\text{arcsec}\right)$, where R is the galactocentric radius. We also present SMBH mass estimates using the Jeans Anisotropic Modelling (JAM) method and Very Large Telescope Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) stellar kinematics. Assuming a cylindrically aligned velocity ellipsoid (JAM$_\text{cyl}$), we infer $M_\text{BH}=(2.52\pm 0.36)\times 10^9$ M$_\odot$, and while assuming a spherically aligned velocity ellipsoid (JAM$_\text{sph}$), we infer $M_\text{BH}=(3.24\pm 0.87)\times 10^9$ M$_\odot$. The SMBH mass assuming a constant $M/L$ is statistically consistent with that of JAM$_\text{sph}$, whereas the mass assuming a linearly varying $M/L$ is consistent with both JAM$_\text{cyl}$ and JAM$_\text{sph}$ (within the uncertainties). Our derived masses are larger than (and inconsistent with) one previous stellar dynamical measurement using the Schwarzschild orbit-superposition method and the same SINFONI kinematics.