NASA Astronomers Zero In on the Number of Hidden Supermassive Black Holes

There are likely more supermassive black holes lurking in the universe than we can see, according to a team of scientists that just ascertained a new estimate for the number of giants concealed from view.

The finding could help scientists understand how supermassive black holes become so large—billions of times the mass of our Sun—and clarify the crucial role that black holes play in galactic evolution.

Black holes have such intense gravitational fields that not even light can escape their vicinity behind a certain point—the black hole’s event horizon. But outside the event horizon, the black hole’s surrounding environment is extremely bright, as it is crowded with a pancake of superheated gas and dust known as an accretion disk.

That material sometimes blocks out light that astronomical observatories would otherwise see. The team found that about 35% of the supermassive black holes they studied are obscured by their surrounding gas and dust. That finding indicates that the number of hidden black holes is larger than previously believed, as earlier searches indicated that around 15% of supermassive black holes were so obscured. The team’s research was published last month in The Astrophysical Journal.

The team reached its conclusions based on data from NASA’s Infrared Astronomical Satellite (IRAS) and the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray observatory. IRAS takes in infrared light (as its name suggests), and the infrared emissions from black hole accretion disks reveal whether the black hole is facing the satellite directly, or if its edge is pointed towards the instrument. After identifying a group of hundreds of initial targets using IRAS, the research team used NuSTAR to confirm edge-on—which is to say obscured—black holes based on its X-ray emissions.

“If we didn’t have black holes, galaxies would be much larger,” said study co-author Poshak Gandhi, an astrophysicist at the University of Southampton, in a NASA Jet Propulsion Laboratory release. “So if we didn’t have a supermassive black hole in our Milky Way galaxy, there might be many more stars in the sky. That’s just one example of how black holes can influence a galaxy’s evolution.”

Furthermore, black holes’ influence can extend far beyond the galaxies in which they reside. Last year, a team of astrophysicists identified the largest-known black hole jets—streams of particles issuing from the object at nearly the speed of light. The jets are dubbed Porphyrion, after a giant of Greek mythology, and are at least 140 times as long as the Milky Way galaxy is wide.

Black holes are crucial engines for galactic evolution, but even these extremely massive objects can evade human detection. The recent research showed how these hidden black holes stay out of sight—and indicates that there are even more of the cosmic juggernauts than we know about.