There are probably more supermassive black holes lurking in the universe than we can see, says a team of scientists who have just established a new estimate of the number of giants hidden from view.
The discovery could help scientists understand how supermassive black holes get so large — billions of times the mass of our Sun — and elucidate the key role black holes play in galactic evolution.
Black holes have such intense gravitational fields that even light cannot escape their vicinity beyond a certain point—the black hole’s event horizon. But beyond the event horizon, the black hole’s environment is extremely bright, because it is packed with a pancake of superheated gas and dust known as the accretion disk.
This material sometimes blocks light that would otherwise be seen by astronomical observatories. The team found that about 35% of the supermassive black holes they studied were obscured by surrounding gas and dust. The discovery suggests that the number of hidden black holes is greater than previously believed, as previous searches have shown that about 15% of supermassive black holes are so obscured. The team’s research was published last month in Astrophysical Journal.
The team reached its conclusions based on data from NASA’s Infrared Astronomy Satellite (IRAS) and the Nuclear Spectroscopic Telescope Array (NuSTAR) X-ray observatory. IRAS receives infrared light (as its name suggests), and the infrared radiation from black hole accretion disks reveals whether the black hole is facing directly toward the satellite or its edge is pointing toward the instrument. After identifying a group of hundreds of initial targets using IRAS, the research team used NuSTAR to confirm edge-on—that is, dimmed—black holes based on their X-ray emissions.
“If we didn’t have black holes, galaxies would be much bigger,” said study co-author Poshak Gandhi, an astrophysicist at the University of Southampton at NASA’s Jet Propulsion Laboratory. rid. “So if we didn’t have a supermassive black hole in our Milky Way galaxy, there could be many more stars in the sky. It’s just one example of how black holes can affect galaxy evolution.”
Furthermore, the influence of black holes 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 leave the object at almost the speed of light. The jets are named Porphyrion, after the giant from Greek mythology, and are at least 140 times the width of the Milky Way galaxy.
Black holes are key drivers of galactic evolution, but even these extremely massive objects can escape human detection. Recent research has shown how these hidden black holes stay out of sight—and suggests that there are even more cosmic powerhouses than we know.