There are probably more massive black holes lurking in the universe than we can see, according to a team of scientists who recently found a new estimate of the number of hidden giants.
The discovery could help scientists understand how massive black holes become—billion times the size of our Sun—and clarify the important role black holes play in galactic evolution.
Black holes have such strong gravitational fields that not even light can escape their surface past a certain point—the black hole’s event horizon. But outside the event horizon, the black hole’s surroundings are extremely bright, as they are densely packed with a super-hot gas and dust packet known as the accretion disk.
Those objects sometimes block the light that astronomers can see. The team found that about 35% of the supermassive black holes they studied were obscured by surrounding gas and dust. The findings indicate that the number of hidden black holes is larger than previously thought, as previous investigations revealed that about 15% of supermassive black holes were hidden. The team’s research was published last month on 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 picks up infrared light (as its name suggests), and the infrared emission from black hole accretion disks reveals whether the black hole is facing the satellite directly, or if its edge is facing the instrument. After identifying a group of hundreds of initial targets using IRAS, the research team used NuSTAR to confirm the boundary—meaning hidden 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 astronomer 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 would be more stars in the sky. That is one example of how black holes can influence the evolution of a galaxy.”
In addition, the influence of black holes can extend beyond the galaxies in which they reside. Last year, a team of astronomers discovered the well-known jets of black holes—streams of particles that shoot out of matter at nearly the speed of light. The jet is called Porphyrion, a name derived from Greek mythology, and its length is at least 140 times the diameter of the Milky Way galaxy.
Black holes are important engines of evolution, but even these massive objects can escape human detection. Recent studies have shown how these hidden black holes remain invisible—and show that there are far more juggernauts than we know.