Supermassive black hole spurring formation of stars instead of devouring them baffles astronomers
Black holes are known to be deadly and destructive, devouring stars and tearing apart other objects. Astronomers were therefore surprised when they recently stumbled across an unusual black hole which is boosting the births of stars in multiple galaxies, and across massive distances, instead of suppressing them.
"This is the first time we've seen a single black hole boost star birth in more than one galaxy at a time. It's amazing to think one galaxy's black hole can have a say in what happens in other galaxies millions of trillions of miles away," said the lead author of the study Roberto Gilli from the National Institute of Astrophysics (INAF) in Bologna, Italy.
"Black holes have a well-earned reputation for being powerful and deadly, but not always. This is a prime example that they sometimes defy that stereotype and can be nurturing instead," says co-author Alessandro Peca, formerly at INAF in Bologna and now a Ph.D. student at the University of Miami.
The supermassive black hole—located in the center of a galaxy about 9.9 billion light-years from Earth—has at least seven neighboring galaxies, says the team.
The black hole, experts say, seems to have enhanced star formation more than one million light-years away—one light-year is equal to six trillion miles.
The researchers estimate that the rates of star formation are between about 100% and 400% higher than typical galaxies with similar masses and distance from Earth. If confirmed, the results would represent the longest distance over which a black hole has triggered the formation of stars.
The discovery was made with NASA's Chandra X-ray Observatory and other telescopes such as the European Southern Observatory's Very Large Telescope (VLT) and the Large Binocular Telescope (LBT).
Astronomers estimate that our galaxy alone contains about 100 million black holes, created when massive stars have collapsed over the past 13 billion years.
Astronomers have seen many cases where a black hole affects its surroundings through what is known as "negative feedback", where it prevents the formation of new stars. This happens when the black hole chokes off star formation because it injects sufficient energy into the hot gas of a galaxy or galaxy cluster—to prevent it from cooling down to make stars.
In this newly discovered collection of galaxies, however, astronomers have found a less common example of "positive feedback," where the black hole is helping to spur star formation, not suppress it. Researchers used X-rays from NASA's Chandra, radio emission detected by the NSF's Karl Jansky Very Large Array (VLA), and optical light from ground-based telescopes to make this discovery. The results have been published in the journal Astronomy and Astrophysics.
According to the team, as hot gas swirls around the black hole, it emits large amounts of X-rays that Chandra detects. The black hole is also the source of radio-wave emission from a jet of high-energy particles—previously detected by scientists with the VLA—that stretches about a million light-years.
Researchers also found a diffuse cloud of X-ray emission surrounding the end of the jet on the left. This X-ray emission is most likely from a gigantic bubble of hot gas heated by the “interaction of the jet's particles with surrounding matter.”
As the “hot bubble expanded and swept through four neighboring galaxies, it could have created a shock wave that compressed cool gas in the galaxies,” causing stars to form, says the study.
According to the team, the four star-forming galaxies at the edge of the X-ray bubble are churning out new stars at rates that are from two to five times higher than their companions and also than what is normally observed in a control sample of field galaxies.
The researchers used a total of six days of Chandra observing time, spread out over five months.
"It's only because of this very deep observation that we saw the hot gas bubble produced by the black hole. By targeting objects similar to this one, we may discover that positive feedback is very common in the formation of groups and clusters of galaxies," says co-author Colin Norman, Johns Hopkins University in Baltimore, Maryland.