We present high angular resolution ( 0 \farcs 3 or 37 \mathrm { pc } ) Atacama Large Millimeter/sub-millimeter Array ( ALMA ) observations of the CO ( 2-1 ) line emission from a central disc in the early-type galaxy NGC 524 .
This disc is shown to be dynamically relaxed , exhibiting ordered rotation about a compact 1.3 \mathrm { mm } continuum source , which we identify as emission from an active supermassive black hole ( SMBH ) .
There is a hole at the centre of the disc slightly larger than the SMBH sphere of influence .
An azimuthal distortion of the observed velocity field is found to be due to either a position angle warp or radial gas flow over the inner 2 \farcs 5 .
By forward-modelling the observations , we obtain an estimate of the SMBH mass of 4.0 ^ { +3.5 } _ { -2.0 } \times 10 ^ { 8 } M _ { \odot } , where the uncertainties are at the 3 \sigma level .
The uncertainties are dominated by the poorly constrained inclination and the stellar mass-to-light ratio of this galaxy , and our measurement is consistent with the established correlation between SMBH mass and stellar velocity dispersion .
Our result is roughly half that of the previous stellar dynamical measurement , but is consistent within the uncertainties of both .
We also present and apply a new tool for modelling complex molecular gas distributions .