The discovery could shed light on how galaxies and black holes grow up together
Astronomer Mark Lacy and colleagues used the Atacama Large Millimeter Array in Chile to observe the universe’s first light, and found evidence of gusts flowing from a type of black hole called a quasar. The wind extends about 228,000 light-years away from the galaxy that surrounds the quasar. Previously, astronomers had seen signs of these winds only about 3,000 light-years from their galaxies.
The result, published November 12 at arXiv.org, could help resolve questions about how black holes can grow with their galaxies, or shut galaxies down for good.
Black holes are best known for gravitationally gobbling everything that veers too close. Paradoxical as it sounds, supermassive black holes can also send material in the opposite direction, driving powerful flows of charged gas and plasma away from their host galaxies.
These black holes are victims of their own success, pulling in more material than they can consume at once. The excess material surrounds black holes in a tight swirling disk, where friction heats it to hundreds of millions of degrees Celsius. The black hole plus that bright disk is a quasar.
All that heat, plus some help from magnetic fields, create great gusts that carry gas and plasma away (SN Online: 3/6/17). “The black hole can’t swallow all of that stuff,” says Lacy, of the National Radio Astronomy Observatory in Charlottesville, Va. “It has to blow some of it out.”
Measuring such winds’ extent and energies could help scientists figure out how material spit out by the black holes might influence the way the galaxies grow and evolve. If the wind doesn’t blow far enough from the galaxy, for example, the material in the gusts could fall back down into the galaxy and be recycled into new stars — or blown back out again (SN: 7/21/18, p. 16).
But if a black hole’s wind is too powerful, it could steal all of a galaxy’s star-forming gas and shut the galaxy down. That could explain why there appears to be a mass limit for galaxies: Most have fewer than 10 trillion times the sun’s mass worth of stars. Theoretical calculations suggest that if a black hole can blow away 1 or 2 percent of the total energy of a quasar in the wind, that would be enough to shut a galaxy down. And that might just happen to be when a galaxy weighs about 10 trillion suns.
To figure out if that actually happens, however, astronomers need to know how far away real black holes’ winds can reach and how much energy they carry.
Lacy and his colleagues observed a quasar called HE 0515-4414, about 268 million light-years away from Earth, to see how the hot gas of its wind scattered photons from the cosmic microwave background, the oldest light in the universe (SN Online: 7/24/18). “It’s almost like the wind casts a shadow,” Lacy says. “You see this hole in the microwave background.”
This phenomenon is called the Sunyaev-Zeldovich effect. Other astronomers predicted in 1999 that the effect could be used to measure the energies and extents of these winds. But ALMA is the first telescope sensitive enough to detect the effect.
In addition to tracking how far HE 0515-4414’s wind blows, the team also measured the gust’s energy. It was much less than expected, about 0.01 percent of the quasar’s total energy. That’s nowhere near enough to explain the galaxy mass limit.
“That doesn’t mean the theory is completely dead,” Lacy says. The ALMA observations suggested that, rather than blowing continuously, the wind blew a large, long-lived bubble of material that can last for many millions of years, longer than most quasars are active. That bubble could keep star-forming material out of the galaxy indefinitely, shutting the galaxy down even without an actively blowing black hole.
“To me that’s the next frontier, to find these ghost outflows hanging around quasars that might be dead,” says astrophysicist Priyamvada Natarajan of Yale University, who wrote the 1999 paper predicting this observation method as a graduate student at the Institute of Astronomy at the University of Cambridge.
“I’m very excited,” she says. “This is the first detection where we can actually measure how much kinetic energy is being transmitted to the environment of the galaxy.” But she cautions that the new study focuses on only one object. Astronomers will need to find more quasar winds before drawing conclusions on how black holes affect their galaxies in general.
M. Lacy et al. Direct detection of quasar feedback via the Sunyaev-Zeldovich effect. arXiv:1811.05023. Posted November 12, 2018.
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