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Active Supermassive Black Holes May Be Engines of Star Formation, Not Just Destruction, Study Suggests
Scientists mapped out the structure of nine bright cores of galaxies located near the Milky Way and revealed common patterns that could shed light on how the universe evolved
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At the heart of some galaxies lie extremely bright spots—far too luminous to be lit by stars alone. Each glowing core outshines the rest of its galaxy combined, thanks to a supermassive black hole that’s devouring matter, pushing out jets and winds, and driving emissions of every type of electromagnetic radiation, from radio waves to gamma rays.
Our galaxy’s own supermassive black hole is quieter. But studying these bright regions, called active galactic nuclei (AGNs), that are sprinkled across the cosmos can teach astronomers about how stars and galaxies form—and possibly shed light on our Milky Way.
Active galactic nuclei “are extreme physics laboratories,” says Niel Brandt, an astrophysicist at Penn State. They are sites where powerful gravity plays out, cloaked in high-temperature plasma. Perhaps most importantly, they include supermassive black holes, which are thought to play a role in cosmic evolution. “We think they have something to do with how galaxies put themselves together,” Brandt adds. “We’ve suspected this for a long time.”
Now, in a step toward understanding these mysterious phenomena, a research team has mapped the dynamics of nine AGNs in our galaxy’s local neighborhood. Their findings, published September 14 in the Astrophysical Journal, suggest that these active regions might promote star formation—contrary to what astronomers have long thought.
“In general, people think overall it’s preventing the star formation in host galaxies,” says study lead author Peixin Zhu, an astrophysicist at the Center for Astrophysics, Harvard and Smithsonian. “But now, what we are seeing is actually the opposite.”
Star formation requires staggeringly low temperatures that are just above absolute zero. But a supermassive black hole ejects jets and waves, which can heat up the surrounding gas—so, it seemed likely that these active cores of galaxies might squash the chance of forming stars.
Quick fact: Accretion disks and jets
A black hole consistently pulls matter in with its intense gravity, creating a hot, bright, spinning disk of debris around it known as an accretion disk. At an active galactic nucleus, magnetic fields eject some of this material in the form of jets or winds, which can travel far beyond the host galaxy.
To find out what’s happening around super-bright AGNs, the team focused on nearby galaxies, because they could get high-resolution views of their centers using the Very Large Telescope in Chile and the Chandra X-Ray Observatory that’s orbiting Earth. Detailed theoretical models, built by the researchers, offered predictions for black hole behavior that they could compare with their observations. In the end, they found a common pattern across the nine AGN-containing galaxies.
The team identified areas of star formation—shaped like rings or arcs—around each of the AGNs, all located somewhere between 2,600 and 19,600 light-years from the center of their galaxies.
“That is a very surprising finding to me, and also, I think, to our community,” Zhu says. Due to their immense gravity, black holes are known for accreting, or pulling in, material. That process could be thought to “take over all the gas” that might otherwise help form stars, she adds. “But then the fact is that in addition to accreting all those things, they also eject things.” The momentum and energy from that ejected material could ultimately promote star formation, although the exact mechanics of how that works is a question for future research.
“We’re seeing that black holes are not just consuming material at the centers of galaxies, but they’re actively reshaping their surroundings,” study co-author and astrophysicist Lisa Kewley, director of the Center for Astrophysics, Harvard and Smithsonian, says in a statement.
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The team also mapped out the charged gas coming from the black holes. This radiation does not extend evenly in all directions; instead, it forms a bicone—a shape like two ice cream cones aimed away from each other, with their points touching at the black hole.
They found that fast shockwaves are also launched out from the black hole, and across the nine galaxies, these shocks extended perpendicular to the bicones of radiation.
That specific geometry came as a second surprise to Zhu. When she initially saw it in her results, she says she felt skeptical. But later, she ran some simulations of black hole dynamics, and to her surprise, they aligned with the observations. “They actually did predict a shockwave going perpendicular to AGN winds or jets,” she says, “and that’s the point I feel like, okay, I’m not seeing something nonsense.”
Brandt, who was not involved with the research, agrees that the perpendicular angle is surprising. “What is the cause of that? That’s what I find to be a little puzzling still,” he says. “This is something that’s a little mysterious.”
With a selection of just nine galaxies, the team can’t draw conclusions about the entire universe. But Zhu says that with future work, they hope to expand that sample. Telescopes such as the Extremely Large Telescope and the Giant Magellan Telescope, both under construction in Chile, are expected to view the cosmos with extremely high resolution.
The AGNs in the study are “very good local laboratories for this kind of work,” Brandt says. “But the real grand cosmic experiment didn’t happen in the nearby universe.” It happened much longer ago, in the far-off universe.
Even if the more distant cosmos can’t be imaged with the same resolution as the team achieved in this study, Brandt wonders whether those objects would add to the story of galactic evolution. “Can we recognize fingerprints of what we’re seeing locally in the distant universe?” he asks.
One of the biggest benefits of the new research, Brandt adds, is that the team has come up with a structural map—a ring or arc of star formation, bicones of radiation and perpendicular shocks—that might be common across AGNs.
Their work offers a sense of order to these bright areas, which are, largely, a cosmic soup of swirling gas, dust, X-rays, charged particles and magnetic fields.
“You kind of look at this and you just think, well, this is just a confusing mixture. It’s just a mess. Maybe we’re not going to be able to sort it out,” Brandt says. “But they’re arguing that maybe you can sort it out. … They are arguing that there is some method to the madness.”