Wandering black holes leave galactic cores

Rather than staying sedately in the cores of their host galaxies, it seems supermassive black holes may also roam their galaxies’ outer regions, thousands of light years from their centres.

Such “wandering” holes are a new class of black holes, says Amy Reines, an astrophysicist at Montana State University, US, who announced the discovery at this week’s meeting of the American Astronomical Society in Hawaii. 

She also is the lead author of a paper in The Astrophysical Journal.

It’s long been known that many galaxies, including our own Milky Way, harbour giant black holes at their hearts. “Almost all galaxies have one,” Reines notes. 

Some of them, in fact, are true monsters. The one in the core of the Milky Way, for example, is about four million times the mass of the Sun, and it is by no means the largest known. That record currently goes to one in a galaxy in the constellation Cetus, whose central black hole is 40 billion times the mass of the Sun. 

Supermassive black holes don’t start out that large. Rather, they grow stepwise as galaxies – and their black holes – collide and merge.

But that begs the question of where the initial supermassive black holes came from. “We don’t know how these black holes got their start,” Reines says. “We don’t know how massive the black hole ‘seeds’ were when they first formed.”

It was the effort to address this mystery that led to the discovery of wandering holes.

Reines’s team was searching for black holes in small galaxies, known as dwarf galaxies, under the theory that these galaxies would have relatively small black holes, helping astrophysicists put limits on the maximum sizes of the enigmatic “seeds”. 

But because they are small, these black holes are also hard to detect. 

So, she turned to one of the world’s largest radio telescopes, the Very Large Array (VLA), in Socorro, New Mexico, using it to zoom in on 111 relatively nearby dwarf galaxies. 

She found that 13 of them “almost certainly” have supermassive black holes – though these holes are only a few hundred thousand times the mass of the Sun – substantially fewer than those in galaxies like our own. 

But she also found that most of them weren’t in their galaxies’ hearts, even though all prior supermassive black holes had been found in their galaxies’ cores. “I was very surprised,” she says. 

But in hindsight, she adds, it shouldn’t have been such a shock. Computer models showed that collisions between galaxies can pull their black holes out of their cores… and the galaxies that showed signs of the recent collisions were the ones most likely not to have their black holes tucked nicely away in their centres.

Furthermore, she says, models show that once a dwarf galaxy’s black hole is pulled out of position, it may never find its way back to the centre. “This is in contrast to giant galaxies that have much more massive black holes that can more easily sink down to the nucleus.”

Meanwhile, her find opens a new window for astronomers seeking additional, and perhaps smaller-yet, supermassive black holes.

“We need to expand our searches throughout the whole galaxy, since a significant fraction of dwarf galaxies might hold black holes that are off-centre.”