WISDOM Project -- XXII. A 5% precision CO-dynamical supermassive black hole mass measurement in the galaxy NGC 383

Abstract

We present a measurement of the supermassive black hole (SMBH) mass of the nearby lenticular galaxy NGC 383, based on Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of the ${}^{12}$CO(2-1) emission line with an angular resolution of $0{.}^{\prime \prime }050\times 0{.}^{\prime \prime }024$ ($\approx 16\times 8$ pc${}^2$). These observations spatially resolve the nuclear molecular gas disc down to $\approx 41,300$ Schwarzschild radii and the SMBH sphere of influence by a factor of $\approx 24$ radially, better than any other SMBH mass measurement using molecular gas to date. The high resolution enables us to probe material with a maximum circular velocity of $\approx 1040$ km/s, even higher than those of the highest-resolution SMBH mass measurements using megamasers. We detect a clear Keplerian increase (from the outside in) of the line-of-sight rotation velocities, a slight offset between the gas disc kinematic (i.e. the position of the SMBH) and morphological (i.e. the centre of the molecular gas emission) centres, an asymmetry of the innermost rotation velocity peaks and evidence for a mild position angle warp and/or non-circular motions within the central $\approx 0{.}^{\prime \prime }3$. By forward modelling the mass distribution and ALMA data cube, we infer a SMBH mass of $(3.58\pm 0.19)\times 10^9$ M${}_{\odot }$ ($1\ddot{I}ƒ$ confidence interval), more precise ($5\%$) but consistent within $\approx 1.4\ddot{I}ƒ$ with the previous measurement using lower-resolution molecular gas data. Our measurement emphasises the importance of high spatial resolution observations for precise SMBH mass determinations.

17 pages, 12 figures. Accepted by MNRAS on January 10th