Astrophys. J. Lett. 822, L28 (2016)

Ever wonder how astronomers measure the mass of a black hole? One way is to map the speed of the rotating disk of gas around it. The Hubble Space Telescope, for example, has mapped many hot disks of ionized gas, but the turbulent motion leads to large measurement uncertainty. Cold molecular gases would be more stable, but they are not visible at optical wavelengths. Fortunately, this situation is ideal for radio telescopes; Aaron Barth et al. have used the Atacama Large Millimeter/submillimeter Array (ALMA) of 66 telescopes in Chile to observe NGC 1332 — a massive elliptical galaxy 73 light years away. They found the central supermassive black hole to be 664 million solar masses, with error bars at the 10% level.

The rotating disk is 800 light years across, but only within the sphere of influence of the black hole (80 light years) is the gravitational force of the black hole dominant. ALMA can see details down to 16 light years, and was able to determine a rotation speed of 500 km s−1. This kind of resolution will help measure other black hole masses with unprecedented precision.