WISDOM Project - XXII. A 5 per cent 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 <inline-formula><tex-math id=TM0001 notation=LaTeX>$^{12}$</tex-math></inline-formula>CO(2-1) emission line with an angular resolution of <inline-formula><tex-math id=TM0002 notation=LaTeX>$0.050{\,\rm arcsec}\times 0.024{\,\rm arcsec}$</tex-math></inline-formula> (<inline-formula><tex-math id=TM0003 notation=LaTeX>$\approx 16\times 8$</tex-math></inline-formula> pc<inline-formula><tex-math id=TM0004 notation=LaTeX>$^2$</tex-math></inline-formula>). These observations spatially resolve the nuclear molecular gas disc down to <inline-formula><tex-math id=TM0005 notation=LaTeX>$\approx 41\,300$</tex-math></inline-formula> Schwarzschild radii and the SMBH sphere of influence by a factor of <inline-formula><tex-math id=TM0006 notation=LaTeX>$\approx 24$</tex-math></inline-formula> 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 <inline-formula><tex-math id=TM0007 notation=LaTeX>$\approx 1040$</tex-math></inline-formula> km s<inline-formula><tex-math id=TM0008 notation=LaTeX>$^{-1}$</tex-math></inline-formula>, 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 <inline-formula><tex-math id=TM0009 notation=LaTeX>$\approx 0.3\,{\rm arcsec}$</tex-math></inline-formula>. By forward modelling the mass distribution and ALMA data cube, we infer an SMBH mass of <inline-formula><tex-math id=TM0010 notation=LaTeX>$(3.58\pm 0.19)\times 10^9$</tex-math></inline-formula> M<inline-formula><tex-math id=TM0011 notation=LaTeX>$_\odot$</tex-math></inline-formula> (<inline-formula><tex-math id=TM0012 notation=LaTeX>$1\sigma$</tex-math></inline-formula> confidence interval), more precise (5 per cent) but consistent within <inline-formula><tex-math id=TM0013 notation=LaTeX>$\approx 1.4\sigma$</tex-math></inline-formula> with the previous measurement using lower resolution molecular gas data. Our measurement emphasizes the importance of high spatial resolution observations for precise SMBH mass determinations.