NASA study finds 35% of supermassive black holes are hidden behind gas and dust, reshaping our understanding of galaxy evolution.
NASA study finds 35% of supermassive black holes are hidden behind gas and dust, reshaping our understanding of galaxy evolution.
Black holes are some of the most mysterious and fascinating objects in the universe. They’re invisible by nature—not even light can escape their immense gravity—but they can also be incredibly bright when they pull in gas and dust, heating it to extreme temperatures. Despite their importance, scientists are still trying to figure out just how many black holes are out there, especially the supermassive ones that lurk at the centers of galaxies. A new NASA study is helping to answer this question by focusing on black holes that are hidden behind thick clouds of gas and dust
Astronomers believe that nearly every large galaxy has a supermassive black hole at its center. These black holes can weigh billions of times more than the Sun. But counting them all is impossible—there could be trillions in the universe. Instead, scientists study smaller samples to estimate the total population. A key part of this is figuring out how many black holes are hidden behind clouds of gas and dust, which block their light from reaching us.
Using multiple NASA telescopes, including the Infrared Astronomical Satellite (IRAS) and the Nuclear Spectroscopic Telescope Array (NuSTAR), researchers conducted a survey to find both hidden and visible supermassive black holes. The study, published in the Astrophysical Journal, found that about 35% of supermassive black holes are heavily obscured by thick clouds of gas and dust. This is a higher percentage than previous estimates, which suggested less than 15% were hidden.
The ratio of hidden to visible black holes is important because it helps scientists understand how these cosmic giants grow and evolve. If black holes grow by consuming gas and dust, then a significant number should be surrounded by thick clouds. The new study supports this idea, suggesting that many black holes are indeed hidden behind these clouds.
But there’s a catch: scientists think the true ratio of hidden to visible black holes should be closer to 50/50, based on models of galaxy formation. If future observations continue to show fewer hidden black holes than expected, it could mean that some of our assumptions about black holes and their role in shaping galaxies need to be revised.
Finding hidden black holes isn’t easy. Most telescopes can only detect black holes that are facing us directly, with their bright central disks visible. But when a black hole is surrounded by a thick torus (a doughnut-shaped cloud) of gas and dust, and viewed edge-on, its light is blocked. However, the torus absorbs some of the light and reemits it as infrared radiation, which can be detected by telescopes like IRAS.
IRAS, which operated for 10 months in 1983, scanned the entire sky and identified hundreds of potential hidden black holes. To confirm these findings, researchers used ground-based telescopes to rule out other sources of infrared light, such as galaxies with high rates of star formation. They then turned to NuSTAR, which detects high-energy X-rays that can penetrate the thick clouds surrounding black holes. By combining data from IRAS and NuSTAR, the team was able to identify and study hidden black holes in unprecedented detail.
Black holes aren’t just cosmic curiosities—they play a crucial role in shaping the galaxies they inhabit. When a black hole consumes too much material at once, it can “cough up” the excess, sending it back into the galaxy. This process can disperse gas clouds that would otherwise form stars, slowing down star formation and influencing how galaxies grow.
“If we didn’t have black holes, galaxies would be much larger,” said Poshak Gandhi, a coauthor of the study and a professor of astrophysics at the University of Southampton. “For example, if the Milky Way didn’t have a supermassive black hole at its center, there might be many more stars in the sky.”
The study highlights the lasting value of telescope archives and the importance of using multiple instruments to study the universe. IRAS, despite being over 40 years old, provided crucial data for identifying hidden black holes, while NuSTAR’s ability to detect high-energy X-rays confirmed their presence.
“It amazes me how useful IRAS and NuSTAR were for this project,” said Peter Boorman, the study’s lead author and an astrophysicist at Caltech. “It shows the benefit of using multiple instruments and wavelengths of light together.”
The study is a step forward in understanding the population of supermassive black holes and their role in the universe. However, there’s still much to learn. Future observations with more advanced telescopes could help refine the ratio of hidden to visible black holes and provide new insights into how these cosmic giants shape the galaxies around them.
As scientists continue to explore the universe, one thing is clear: black holes, whether hidden or visible, are key players in the cosmic story. And thanks to NASA’s telescopes, we’re getting closer to uncovering their secrets.
