A team of scientists from the University of Utah and the Smithsonian Astrophysical Observatory has found a new explanation for the growth of supermassive black holes in the center of most galaxies.
An artist’s conception of a supermassive black hole (left) capturing one star (bluish) from a pair of binary stars (Benjamin Bromley / University of Utah)
The study, published online today in the Astrophysical Journal Letters, suggests that supermassive black holes repeatedly capture and swallow single stars from pairs of stars that wander too close.
“Using new calculations and previous observations of our own Milky Way and other galaxies, we found black holes grow enormously as a result of sucking in captured binary star partners,” said Prof. Benjamin Bromley of the University of Utah, a lead author of the study. “I believe this has got to be the dominant method for growing supermassive black holes. There are two ways to grow a supermassive black hole: with gas clouds and with stars. Sometimes there’s gas and sometimes there is not. We know that from observations of other galaxies. But there are always stars.”
“Our mechanism is an efficient way to bring a star to a black hole,” Prof. Bromley explained. “It’s really hard to target a single star at a black hole. It’s a lot easier to throw a binary at it, just as it’s more difficult to hit a target using a slingshot, which hurls a single stone, than with a bola, which hurls two weights connected by a cord.”
“A binary pair of stars orbiting each other is essentially a single object much bigger than the size of the individual stars, so it is going to interact with the black hole more efficiently. The binary doesn’t have to get nearly as close for one of the stars to get ripped away and captured.”
But to prove the theory will require more powerful telescopes to find three key signs: large numbers of small stars captured near supermassive black holes, more observations of stars being “shredded” by gravity from black holes, and large numbers of “hypervelocity stars” that are flung from galaxies at more than 1 million mph when their binary partners are captured.
Small black holes result from the collapse of individual stars. But the centers of most galaxies, including our own Milky Way, are occupied by what are popularly known as supermassive black holes that contain mass ranging from 1 million to 10 billion stars the size of our Sun.
Astrophysicists have long debated how supermassive black holes grew during the 14 billion years since the Universe began in a great expansion of matter and energy named the Big Bang. One side believes black holes grow larger mainly by sucking in vast amounts of gas, the other side says they grow primarily by capturing and sucking in stars.
“The hypervelocity stars we see come from binary stars that stray close to the galaxy’s massive black hole,” Prof. Bromley said. “The hole peels off one binary partner, while the other partner – the hypervelocity star – gets flung out in a gravitational slingshot.”
“We put the numbers together for observed hypervelocity stars and other evidence, and found that the rate of binary encounters would mean most of the mass of the galaxy’s black hole came from binary stars,” he added. “We estimated these interactions for supermassive black holes in other galaxies and found that they too can grow to billions of solar masses in this way.”
The study looked at each step in the process of a supermassive black hole eating binary stars, and calculated what would be required for the process to work in terms of the rates at which hypervelocity stars are produced, binary partners are captured, the captured stars are bound to the black hole in elongated orbits and then sucked into it.
The team then compared the results with actual observations of supermassive black holes, stars clustering near them and tidal disruption events in which black holes in other galaxies are seen to shred stars while pulling them into the hole.
“It fits together, and it works,” Prof. Bromley said. “When we look at observations of how stars are accumulating in our galactic center, it’s clear that much of the mass of the black hole likely came from binary stars that were torn apart.”
The scientist refers to the process of a supermassive black hole capturing stars from binary pairs as “filling the bathtub.” Once the tub – the area near the black hole – is occupied by a cluster of captured stars, they go “down the drain” into the black hole over millions of years. The study shows the “tub” fills at about the same rate it drains, meaning stars captured by a supermassive black hole eventually are swallowed.
When the scientists considered the number of stars near the Milky Way’s center, their speed and the odds they will encounter the supermassive black hole, they estimated that one binary star will be torn apart every 1,000 years by the hole’s gravity. During the last 10 billion years, that would mean the Milky Way’s supermassive black hole ate 10 million solar masses – more than enough to account for the hole’s actual size of 4 million solar masses.
“We found a wide range of black hole masses can be explained by this process,” Prof. Bromley concluded.
