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

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 NGC 4526. 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 local galaxies with massive black holes will be observable, and that given sufficient surface brightness sensitivity one could measure the mass of a > ∼ 4×10 M⊙ black hole at any redshift.