A figure of merit for black hole mass measurements with molecular gas

TL;DR

This paper introduces a figure of merit for estimating black hole masses using molecular gas kinematics, showing that accurate measurements are possible even with resolutions twice the black hole's sphere of influence, and discusses future observational prospects.

Contribution

It presents a new figure of merit for black hole mass measurements with molecular gas and analyzes the observational requirements and future potential with ALMA.

Findings

Black hole masses can be estimated with data resolving ~2 times the sphere of influence.

Reanalysis confirms lower resolution data can still yield accurate masses.

Future ALMA observations will enable measurements of black holes in over 10^5 galaxies.

Abstract

In this work we discuss the technique of using molecular gas kinematics (or the kinematics of any dynamically cold tracer) to estimate black hole masses. We present a figure of merit that will be useful in defining future observational campaigns, and discuss its implications. We show that, in principle, one can estimate black-hole masses using data that only resolve scales ~2 times the formal black hole sphere of influence, and confirm this by reanalysing lower resolution observations of the molecular gas around the black hole in NGC4526. We go on to discuss the effect that angular resolution, velocity resolution and the depth of the galaxies potential have on the ability to estimate black hole masses, and conclude by discussing prospects for the future. Once ALMA is fully operational, we find that over 10^5 local galaxies with massive black holes will be observable, and that given sufficient surface brightness sensitivity one could measure the mass of a >4x10^8 Msun black hole at any redshift.