  The masses of the supermassive black-holes found in galaxy bulges are
correlated with a multitude of galaxy properties, leading to suggestions that
galaxies and black-holes may evolve together. The number of reliably measured
black-hole masses is small, and the number of methods for measuring them is
limited, holding back attempts to understand this co-evolution. Directly
measuring black-hole masses is currently possible with stellar kinematics (in
early-type galaxies), ionised-gas kinematics (in some spiral and early-type
galaxies) and in rare objects which have central maser emission. Here we report
that by modelling the effect of a black-hole on the kinematics of molecular gas
it is possible to fit interferometric observations of CO emission and thereby
accurately estimate black hole masses. We study the dynamics of the gas in the
early-type galaxy NGC4526, and obtain a best fit which requires the presence of
a central dark-object of 4.5(+4.2-3.0)x10^8 Msun (3 sigma confidence limit).
With next generation mm-interferometers (e.g. ALMA) these observations could be
reproduced in galaxies out to 75 megaparsecs in less the 5 hours of observing
time. The use of molecular gas as a kinematic tracer should thus allow one to
estimate black-hole masses in hundreds of galaxies in the local universe, many
more than accessible with current techniques.
