Multiple NASA telescopes recently helped scientists search the sky for supermassive black holes — those up to billions of times heavier than the Sun. The new survey is unique because it was as likely to find massive black holes that are hidden behind thick clouds of gas and dust as those that are not.

Astronomers now think that every large galaxy in the universe has a supermassive black hole at its center. But testing this hypothesis is difficult because researchers can’t hope to count the billions or even trillions of supermassive black holes thought to exist in the universe, and instead have to extrapolate from smaller samples to learn about the larger population. So accurately measuring the ratio of hidden supermassive black holes in a given sample will help scientists better estimate the total number of supermassive black holes in the universe.

The new study published in the Astrophysical Journal found that about 35% of supermassive black holes are heavily obscured, meaning the surrounding clouds of gas and dust are so thick they even block low-energy X-ray light. Comparable searches have previously found less than 15%. Scientists think the true split should be closer to 50/50 based on models of how galaxies grow. But if observations continue to show a lower percentage, scientists will need to adjust some key ideas they have about supermassive black holes and the role they play in shaping galaxies.

Hidden Treasure

Although black holes are inherently dark — not even light can escape their gravity — they can also be some of the brightest objects in the universe: When gas gets pulled into orbit around a supermassive black hole, like water circling a drain, the extreme gravity creates such intense friction and heat that the gas reaches hundreds of thousands of degrees and radiates so brightly it can outshine all the stars in the surrounding galaxy.

The clouds of gas and dust that surround and replenish the bright central disk may roughly take the shape of a torus, or doughnut. If the doughnut hole is pointed toward Earth, the bright central disk within it is visible; if the doughnut is edge-on, the disk is obscured. Most telescopes can rather easily identify face-on supermassive black holes, but not edge-on ones.

But there’s an exception to this that the authors of the new paper took advantage of: The doughnut absorbs light from the central source and reemits lower-energy light in the infrared range, or wavelengths slightly longer than what human eyes can detect. Essentially, the doughnuts glow in infrared.

These wavelengths of light were detected by NASA’s Infrared Array Survey, or IRAS, mission, which operated for 10 months in 1983 and was managed by NASA’s Jet Propulsion Laboratory in Southern California. A survey telescope that imaged the entire sky, IRAS was able to see the infrared emissions from the clouds surrounding supermassive black holes. Most importantly, it could spot edge-on and face-on black holes equally well.

IRAS caught hundreds of initial targets. Some of them turned out to not be heavily obscured black holes, but galaxies with high rates of star formation that emit a similar infrared glow. So, the team used ground-based, visible-light telescopes to identify the latter and separate them.

To confirm edge-on, heavily obscured black holes, the researchers relied on NASA’s NuSTAR (Nuclear Spectroscopic Telescope Array), an X-ray observatory also managed by JPL. X-rays are radiated by some of the hottest material around the black hole. Lower-energy X-rays will be absorbed by the surrounding clouds of gas and dust, while the higher-energy X-rays observed by NuSTAR can penetrate and scatter off the clouds. This can take hours of observation, so scientists working with NuSTAR first need a telescope like IRAS to tell them where to look.

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

Numerical Advantage

Determining the number of hidden black holes compared to non-hidden ones can help scientists understand how these black holes get so big. If they grow by consuming material, then a significant number of black holes should be surrounded by thick clouds and potentially obscured. Boorman and his coauthors say this first unbiased look at the population supports this hypothesis.

In addition, black holes influence the galaxies they live in, mostly by impacting how galaxies grow. This happens because black holes surrounded by massive clouds of gas and dust can consume vast — but not infinite — amounts of material. If too much falls toward a black hole at once, the black hole starts coughing up the excess and firing it back out into the galaxy. That can disperse gas clouds within the galaxy where stars are forming, slowing the rate of star formation in the galaxy.

“If we didn’t have black holes, galaxies would be much larger,” said Poshak Gandhi, a professor of astrophysics at Southampton University in the United Kingdom and a coauthor on the new study. “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.”

More About NuSTAR

NuSTAR launched on June 13, 2012. A Small Explorer mission led by Caltech in Pasadena, California, and managed by JPL for NASA’s Science Mission Directorate in Washington, it was developed in partnership with the Danish Technical University (DTU) and the Italian Space Agency (ASI). The telescope optics were built by Columbia University, NASA’s Goddard Space Flight Center in Greenbelt, Maryland, and DTU. The spacecraft was built by Orbital Sciences Corp. in Dulles, Virginia. The NuSTAR mission’s operations center is at the University of California, Berkeley, and the official data archive is at NASA’s High Energy Astrophysics Science Archive Research Center. ASI provides the mission’s ground station and a mirror data archive. Caltech manages JPL for NASA.

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Calla Cofield
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