At the heart of every galaxy there seems to be a supermassive black hole; that much astronomers are pretty sure about. What’s much less well understood is the possible connection between the mass of the central black hole and the size of the galaxy’s bulge; there are hints that supermassive black holes and galaxies might co-evolve, but the details are unknown. One of the barriers towards understanding this possible relationship is the difficulty in measuring black hole masses. It’s not impossible to measure the mass of a supermassive black hole: you can track stars as they orbit the black hole, for example, and use their motion to deduce the mass. Or, even better, you can track the motion of ionised gas clouds. In a few rare cases, as I mentioned in Measuring the Universe, it’s possible to observe maser emission from the central regions and from this measure a mass. These methods only work for nearby galaxies, however.
Timothy Davis, Martin Bureau, Michele Cappellari, Marc Sarzi and Leo Blitz, in a paper published in the 30~January~2013 issue of Nature (A black-hole mass measurement from molecular gas kinematics in NGC4526) have developed a new technique for measuring the mass of a supermassive black hole and used it to estimate the mass of the central black hole in NGC4526.
The team’s idea is to measure the motion of molecular gas clouds by observing emission from carbon monoxide (CO). They tested their idea by observing the CO(2-1) emission line from gas in the lenticular galaxy NGC 4526, which is about 17 Mpc away from us in the Virgo cluster. The wavelength from such CO emission is 1.3 mm, which means that millimetre-wavelength telescopes are required. The team used the 23-telescope Combined Array for Research in Millimetre Astronomy (CARMA) to observe the emission, and from their observations deduced a central black hole mass of about 450 million solar masses.
What’s interesting about this paper is not the result itself, but the possibilities it opens. The team needed 100 hours of observing time with CARMA to get their result. With ALMA, however, the same job would take less than an hour: observations become possible that were previously impractical. When it becomes fully operational ALMA, using this technique, may well provide us with an understanding of how galaxies and their supermassive black holes live together.