How Do Scientists Weigh a Supermassive Black Hole?

A new method puts the mass of one black hole at 140 million times the mass of our Sun

galaxy NGC 1097
An image of the galaxy NGC 1097, home to the black hole researchers just weighed ALMA (NRAO/ESO/NAOJ), K. Onishi; NASA/ESA Hubble Space Telescope, E. Sturdivant; NRAO/AUI/NSF

Black holes are immensely strange structures — so massive that if it gets too close, even light can’t escape their gravitational pull. But bit by bit astronomers are learning more about them, including the supermassive black holes that live at the center of every large galaxy. Recently, a team actually weighed one.

Astronomers are interested in figuring out exactly how big black holes are because evidence indicates that the evolution of galaxies and the formation of black holes are intimately related. 

Black holes are so named because they can’t be seen. This makes them hard to study directly. Instead researchers have to look at the environment around the black hole to pick up clues. A press release by Charles Blue at the National Radio Astronomy Observatory (NRAO) explains exactly how researchers measured the mass of a supermassive black hole in the center of a spiral galaxy named NGC 1097.

To gather the information they needed for the measurement, the team used the huge Atacama Large Millimter/submillimeter Array (ALMA), a collection of 66 radio telescopes scattered across the Chajnantor plateau in northern Chili, at approximately 16,400 feet above sea level. The high dry air there lets astronomers see better into space. 

To measure the mass of the black hole at the center of our our own Milky Way, Sagittarius A*,​ researchers in the past have tracked the movement of stars near the black hole. But NGC 1097 is 45 million light-years away, too far to measure its stars’ movements reliably. It’s also a barred, spiral galaxy, a type that spins in a way that makes measuring things around it difficult. Instead, the astronomers studied the overall distribution of two molecules, hydrogen cyanide (HCN) and formylium (HCO+) in that distant galaxy, reports Ian O’Neill for Discovery News. Then they matched those observations with computer simulations to figure out how much black hole actually weighs. 

What they found was that the spiral galaxy’s black hole is 140 million times more massive than Earth’s Sun. In piddling comparison, Sagittarius A* is just a few million times more massive than the Sun. The team published their findings in the Astrophysical Journal.

"It’s exciting to think that we can now apply this same technique to other similar galaxies and better understand how these unbelievably massive objects affect their host galaxies," Kartick Sheth, an astronomer and co-author on the paper says in the press release from NRAO. 

As impressive as this black hole’s mass is, there are much larger ones out there. There’s one black hole that may be as large as 17 billion times the Sun’s mass. Now that researchers can measure even more black holes in different kinds of galaxies, even that record could be shattered.