The closest quasar to Earth might harbor a rare pair of supermassive black holes. The dark duo is probably left over from a galaxy collision, researchers report, and offers a way to test theories about gravity and the growth of galaxies.
Black hole couples should pop up in the center of pretty much every galaxy at some point, as galaxies grow by colliding with one another. This galactic cannibalism eventually brings together the black holes that reside in their centers. “If we cannot find any binary black holes in the centers of big galaxies, that means our understanding of galaxy formation may be wrong,” says Youjun Lu, an astrophysicist at the National Astronomical Observatories of the Chinese Academy of Sciences in Beijing.
Such a pair might be snuggled up in the heart of Markarian 231, a galaxy roughly 590 million light-years away in the constellation Ursa Major, Lu and colleagues report in the Aug. 20 Astrophysical Journal.
Mrk 231 hosts a quasar, a blazing beacon powered by superheated gas spiraling around a gargantuan black hole. Quasars emit light at many frequencies, but the one in Mrk 231 is missing some ultraviolet light. The absent radiation might be due to a gap in the quasar disk cleared out by a second, smaller black hole, says Lu.
“It’s a plausible explanation of the data,” says George Djorgovski, an astrophysicist at Caltech, “but by itself is not compelling evidence.” Mrk 231 is a complex place, he says, and the large number of variables in the underlying equations can be tuned to support almost any conclusion.
Until recently, only a handful of possible pairs of supermassive black holes were known. Djorgovski and colleagues, who earlier this year discovered another tight black hole pairing (SN: 2/7/15, p. 9), identified 111 candidate couples by looking for rhythmic changes in quasar brightness, they report August 25 in Monthly Notices of the Royal Astronomical Society. “It’s totally possible that Mrk 231 is one of these systems,” he says. But researchers need to first spend a few years watching for similar fluctuations.
Black hole pairs eventually merge to become one. Such cosmic unions are great laboratories for testing general relativity, Einstein’s century-old theory that describes how gravity works. As the doomed duo spiral together, gravitational waves ripple out through spacetime — waves that astrophysicists hope to directly detect in the coming years. None of the efforts to detect these waves could sense a gravitational flutter from Mrk 231 — the alleged black holes are too far apart. A census of binary black holes, however, could provide a way to check some of the assumptions underlying attempts to sniff out black holes that are more tightly huddled up.
Such a census can also reveal what role these cataclysmic collisions play in the development of galaxies. Quasars stunt star formation for tens of millions of years as they blast their host galaxies with radiation and fountains of charged particles. Black hole mergers release a comparable amount of energy in one pulse. “That’s like 10 billion supernovas going off at once,” says Djorgovski. “That can do some serious damage to the host galaxy.”