# Black hole mass measurement using molecular gas kinematics: what ALMA can do

## TL;DR

This paper evaluates the capabilities and limitations of ALMA radio interferometry in measuring supermassive black hole masses through molecular gas kinematics, considering resolution, systematic errors, and galaxy profile biases.

## Contribution

It provides a detailed analysis of the resolution requirements and systematic effects impacting SMBH mass measurements with ALMA, including validation with simulations.

## Key findings

- Higher S/N reduces systematic errors in SMBH mass estimates.
- Poor galaxy profile resolution can bias SMBH mass by an order of magnitude.
- Systematic effects are manageable if spatial and velocity scales are well-resolved.

## Abstract

We study the limits of the spatial and velocity resolution of radio interferometry to infer the mass of supermassive black holes (SMBHs) in galactic centres using the kinematics of circum-nuclear molecular gas, by considering the shapes of the galaxy surface brightness profile, signal-to-noise ratios (S/Ns) of the position-velocity diagram (PVD) and systematic errors due to the spatial and velocity structure of the molecular gas. We argue that for fixed galaxy stellar mass and SMBH mass, the spatial and velocity scale that need to be resolved increase and decrease, respectively, with decreasing \sersic\ index of the galaxy surface brightness profile. We validate our arguments using simulated PVDs for varying beam size and velocity channel width. Furthermore, we consider the systematic effects on the inference of the SMBH mass by simulating PVDs including the spatial and velocity structure of the molecular gas, which demonstrates that their impacts are not significant for a PVD with good S/N unless the spatial and velocity scale associated with the systematic effects are comparable to or larger than the angular resolution and velocity channel width of the PVD from pure circular motion. Also, we caution that a bias in a galaxy surface brightness profile owing to the poor resolution of a galaxy photometric image can largely bias the SMBH mass by an order of magnitude. This study shows the promise and the limit of ALMA observations for measuring SMBH mass using molecular gas kinematics and provides a useful technical justification for an ALMA proposal with the science goal of measuring SMBH mass.