Wandering Black Holes Take Food With Them

Like well-prepared travelers, some wandering black holes take their food with them as they journey through—or even beyond—their host galaxies.

Some supermassive black holes at the centers of large galaxies are thought to be ringed by swirling belts of hot gas, called “accretion” disks, upon which they feed. Black holes that have large quantities of matter falling into them in this way are called “quasars,” and they emit so much energy that they outshine all the stars in their host galaxies.

When two large galaxies merge, their black holes also coalesce. If the black holes have unequal masses, their merger causes powerful gravitational radiation to be emitted in a single direction. Like a mighty shotgun blast, the “radiation kick” causes the black hole or quasar to recoil, at speeds of up to 10 million miles per hour (16 million km/h).

“It will not be a quasar situated in the middle of the merger galaxy. It will be a displaced quasar,” explained Avi Loeb, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics.

Despite recent advances in computer modeling of black-hole mergers, it was unclear whether recoiling quasars could hang on to their accretion disks (the food) during ejection from the galactic center.

New calculations by Loeb suggest they can. Loeb predicts that so long as the accretion disk spins around a black hole faster than the black hole’s ejection speed, the disk will follow the black hole on its journey.

This is an easy requirement for most quasars, the accretion disks of which can spin at close to the speed of light. “The matter is so tightly bound that it follows the black hole like a herd of sheep around a shepherd,” Loeb said.
Loeb estimates a displaced quasar can travel about 30,000 light-years from its galactic center before exhausting its accretion disk. Without a steady food supply, the quasar stops radiating energy and becomes a quiescent black hole.

“Even if you don’t feed it, the [black hole] will survive forever basically,” Loeb told SPACE.com.

If a wandering black hole is not traveling at high enough speeds to escape its host galaxy entirely, it will eventually fall back to the galactic center in about 100 million years, Loeb said.

“It’s like throwing a stone up in the air,” he said. “If you don’t throw it at a high enough speed, it comes back.”

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Staff Writer

Ker Than is a science writer and children's book author who joined Space.com as a Staff Writer from 2005 to 2007. Ker covered astronomy and human spaceflight while at Space.com, including space shuttle launches, and has authored three science books for kids about earthquakes, stars and black holes. Ker's work has also appeared in National Geographic, Nature News, New Scientist and Sky & Telescope, among others. He earned a bachelor's degree in biology from UC Irvine and a master's degree in science journalism from New York University. Ker is currently the Director of Science Communications at Stanford University.