ALMA Helps Scientists Measure Black Hole’s Mass With Unprecedented Precision

The fact that black holes are extremely massive is quite well-known. However, determining the exact mass of these superdense bodies has long proven elusive as it requires a precise measurement of the strength of their gravitational pull on the stars and clouds of gas that encircle them.

Until now, these measurements have relied on mapping the rotation of ionized gas discs surrounding the black hole that glow at visible wavelengths and can thus be observed by optical telescopes such as the Hubble. However, ionized discs tend to be much more turbulent than their colder counterparts, leading to a greater uncertainty and lower precision when measuring a black hole’s mass.

Now, a team of astronomers using data gathered by the Atacama Large Millimeter/submillimeter Array in Chile have found a way to significantly scale up this precision.

ALMA, as its name suggests, is capable of observing millimeter and submillimeter wavelengths, and can be used to detect non-visible radiation emitted by cold interstellar gas swarming around supermassive black holes that lie at the centers of their host galaxies.

Once the radiation is detected, the molecular gas disc’s speed of rotation within the black hole’s “sphere of influence” — the region where the black hole's gravity is dominant — can be accurately determined. This can then be used to quantify the black hole’s gravitational pull, which can be used to accurately determine its mass.

“This is the first time that ALMA has probed the orbital motion of cold molecular gas well inside the gravitational sphere of influence of a supermassive black hole,” Aaron Barth, a professor of physics and astronomy at the University of California and the lead author of a study published in the Astrophysical Journal Letters, said in a statement. “We’re directly viewing the region where the cold gas is responding to the black hole’s gravitational pull. This is an exciting milestone for ALMA and a great demonstration of its high-resolution capability.”

The black hole researchers focused on for this particular study lies at the center of NGC 1332 — a massive elliptical galaxy approximately 73 million light-years from Earth. ALMA observations revealed that the galaxy contained a flattened disc of cold molecular gas rotating around its center and extending nearly 800 light-years from the galactic nucleus.

After measuring the disc’s rotation within an 80 light-year radius — the black hole’s sphere of influence — scientists determined, with an unprecedented measurement uncertainty of just 10 percent, that NGC 1332’s black hole has a mass 660 million times greater than our sun.

“This black hole, though individually massive, accounts for less one percent of the mass of all the stars in the galaxy,” Barth said. “Most of a galaxy's mass is in the form of dark matter and stars, and on the scale of an entire galaxy, even a giant black hole is just a tiny speck in the centre.”