Most picture the night sky as an endless sea of twinkling stars. But if your eyes could see the universe in X-rays, you’d spot thousands upon thousands of bright points representing giant black holes feeding on gas and dust. A new multi-organizational study published in the Astrophysical Journal suggests we may have missed nearly half of these feasting supermassive black holes since many hide behind thick clouds of gas and dust that block most of the light that black holes emit from their accretion disks.

“If our eyes were able to detect X-rays, the sky would be full of dots,” said Peter Boorman, an astrophysicist at the California Institute of Technology, speaking at an American Astronomical Society meeting. “And every single one of those dots would be an accreting supermassive black hole.”

Supermassive black holes grow by pulling in clouds of dust and gas, releasing electromagnetic radiation in the process. Ironically, the same dust that feeds these black holes can also hide them from view, especially in optical or lower-energy observations.

Why So Many Are Hidden

The clouds of gas and dust surrounding and replenishing the bright central disk of a black hole may roughly take the shape of a torus, or doughnut. If the doughnut hole is facing toward Earth, the bright central disk within it is visible; if the doughnut is seen edge-on, the disk is obscured.

Most telescopes can easily identify face-on supermassive black holes, though not edge-on ones. But there’s an exception to this: The torus absorbs light from the central source and reemits lower-energy light in the infrared range, making the doughnuts glow in infrared.
This discovery builds on recent observations from NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array) mission and infrared data gathered by a decades-old now-defunct IRAS satellite (the Infrared Astronomical Satellite) which operated for just 10 months in 1983.

Scientists found that up to 30% to 50% more black holes are obscured by dust and gas than previously believed. This means we’ve been undercounting them for years, because our telescopes couldn’t easily spot objects buried behind thick curtains of cosmic debris.

“It amazes me how useful IRAS and NuSTAR were for this project, especially despite IRAS being operational over 40 years ago,” said Boorman who led the study. “I think it shows the legacy value of telescope archives and the benefit of using multiple instruments and wavelengths of light together.”

NuSTAR helped uncover these “secret” black holes by detecting very high-energy X-rays that can pierce the thick clouds surrounding them. Meanwhile, IRAS provided clues in the form of infrared light, which is given off by the dust hiding the black holes. Combining these data sets allowed Boorman and his colleagues to locate black holes that had previously evaded astronomers’ notice.

A black hole hiding behind gas and dust can still shine brightly, but only at wavelengths of light that can escape that dusty shroud, such as certain X-rays or infrared emissions. Researchers say the next step is expanding this search to distant galaxies. That way, scientists can learn how black holes (and their galaxies) grew and evolved across the universe’s history.

Why Does It Matter?

Supermassive black holes, though tiny compared to the enormous galaxies they live in, can greatly affect their surroundings. As they devour gas and dust, they sometimes eject jets of high-speed material back into space. These jets can clear out the very stuff needed to create new stars, influencing how many stars a galaxy can form. Additionally, determining the number of hidden black holes compared to nonhidden ones can help scientists understand how these black holes get so big.

“If we didn’t have black holes, galaxies would be much larger,” said Poshak Gandhi, an astrophysicist at the University of Southampton and coauthor. “So if we didn’t have a supermassive black hole in our Milky Way galaxy, there might be many more stars in the sky.”

By finding more of these black holes, scientists say they can better understand how these mysterious objects grow—and how they help or hinder the formation of new stars in their host galaxies.

“For years, we’ve been collecting pieces of this cosmic puzzle,” Gandhi said. “Now we’re starting to see the broader picture, and it’s revealing that black holes play an even bigger role in galaxy evolution than we realized.”