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

Abstract

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 NGC 0383 (radio source 3C 031). This measurement is based on Atacama Large Millimeter/sub-millimeter Array (ALMA) cycle 4 and 5 observations of the ¹²CO(2-1) emission line with a spatial resolution of 58 × 32 pc² (0.18 arcsec × 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. NGC 0383 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) × 10⁹ M_⊙, a F160W-band stellar mass-to-light ratio that varies from 2.8 ± 0.6 M_⊙/L_{⊙ , F160W} in the centre to 2.4 ± 0.3 M_⊙/L_{⊙ , F160W} at the outer edge of the disc and a molecular gas velocity dispersion of 8.3 ± 2.1 km s^-1(all 3σ 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 (≈ 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.