WISDOM project – V. Resolving molecular gas in Keplerian rotation around the supermassive black hole in NGC 0383

As part of the mm-Wave Interferometric Survey of Dark Object Masses (WISDOM), we present a measurement of the mass of the supermassive black hole (SMBH) in the nearby early-type galaxy NGC0383 (radio source 3C 031). This measurement is based on Atacama Large Millimeter/sub-millimeter Array (ALMA) cycle 4 and 5 observations of the (CO)-C-12(2-1) emission line with a spatial resolution of 58 x 32 pc(2) (0.18 arcsec x 0.1 arcsec). This resolution, combined with a channel width of 10 km s(-1), allows us to well resolve the radius of the black hole sphere of influence (measured as R-SOI = 316 pc = 0.98 arcsec), where we detect a clear Keplerian increase of the rotation velocities. NGC0383 has a kinematically relaxed, smooth nuclear molecular gas disc with weak ring/spiral features. We forward model the ALMA data cube with the KINEMATIC MOLECULAR SIMULATION (KinMS) tool and a Bayesian Markov Chain Monte Carlo method to measure an SMBH mass of (4.2 +/- 0.7) x 10(9)M(circle dot), a F160W-band stellar mass-to-light ratio that varies from 2.8 +/- 0.6M(circle dot)/L-circle dot,L- (F160W) in the centre to 2.4 +/- 0.3M(circle dot)/L-circle dot,L- F160W at the outer edge of the disc and a molecular gas velocity dispersion of 8.3 +/- 2.1 kms(-1)(all 3 sigma uncertainties). We also detect unresolved continuum emission across the full bandwidth, consistent with synchrotron emission from an active galactic nucleus. This work demonstrates that low-J CO emission can resolve gas very close to the SMBH (approximate to 140 000 Schwarzschild radii) and hence that the molecular gas method is highly complimentary to megamaser observations, as it can probe the same emitting material.