# WISDOM project - IV. A molecular gas dynamical measurement of the supermassive black hole mass in NGC 524

## TL;DR

This study uses high-resolution ALMA observations of molecular gas in NGC 524 to dynamically measure the supermassive black hole's mass, providing a new approach and comparison with previous stellar dynamical estimates.

## Contribution

It introduces a novel molecular gas dynamical method for SMBH mass measurement and applies it to NGC 524, demonstrating its effectiveness and consistency with established correlations.

## Key findings

- SMBH mass estimated at approximately 4.0 x 10^8 solar masses.
- The molecular gas disc is dynamically relaxed with ordered rotation.
- The measurement aligns with the SMBH mass-stellar velocity dispersion relation.

## Abstract

We present high angular resolution (0.3" or 37 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 1.3mm 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 2.5". By forward-modelling the observations, we obtain an estimate of the SMBH mass of $4.0^{+3.5}_{-2.0}\times10^8\,\mathrm{M_\odot}$, 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.