Black holes are big — no one is disputing that. However, the question of exactly how big a black hole is can change the way we understand the formation of the universe and evolution of galaxies. Astronomers from the University of California, Irvine used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to analyze a supermassive black hole like never before, and were able to determine its mass far more accurately than previous measurements.
The black hole in question is at the center of NGC 1332, a giant elliptical galaxy about 73 million light years away from Earth. Most galaxies have giant black holes in the center, but only one in ten elliptical galaxies have disks of cold molecular gas and dust that orbit the black hole and allow for more accurate measurements. This is where ALMA directed its 66 radio antennas. There’s no way to look at the back hole directly, but it’s this spinning vortex of matter that can give away its true size.
It’s a common misconception (aided by occasional hyperbolic descriptions) that black holes consume any and all matter and energy around them. While they do have tremendous gravitational pull, that doesn’t mean everything falls into them. The back hole in NGC 1332 has a disk roughly 800 light years in diameter. The innermost 80 light years is the most important area where the black hole’s gravity is the predominant force (known as the sphere of influence).
The team focused on tracking the radio wave emissions from carbon monoxide molecules in the disk. In September 2014 ALMA was used to probe the black hole for carbon monoxide signatures in the sphere of influence, then the same scan was run a year later at a higher resolution. The data revealed structures as small as 16 light years across — not bad for a different galaxy. The instrument reported that the disk had a maximum rotational speed of 500km per second (over 1.1 million miles per hour). Mapping the rotation of the disk gave the team the black hole’s precise mass — 660 solar masses.
Astronomers believe this method to be more accurate than past visible spectrum measurements based on hotter ionized gas around other black holes. The turbulent nature of such matter makes rotational analysis much less accurate. The team is currently analyzing ALMA data from several more elliptical galaxies in the study, and six more are lined up for future observation.