Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse
Object type: Middleweight black hole
Mass: 20,000 suns
Location: 290 million light years from Earth
HLX-1 was a big fish in a small pond. As the heaviest black hole in a dwarf-sized galaxy, it was master of its domain. It didn't care that it was a fraction of a per cent the size of the central black holes in big, fancy galaxies like the Milky Way.
At 20,000 times the mass of the sun, it still far outweighed the sun-sized black holes buzzing around it like gnats. Its host galaxy also provided it with a steady supply of gas, dust and even smaller black holes to feed on.
But all that changed 200 million years ago, when its little pond emptied into a much bigger sea. Its small host galaxy got too close to a giant galaxy called ESO?243-49 and was gravitationally ripped to shreds. Most of its host's gas and dust was torn away from HLX-1, which was left orphaned inside the bigger bully. Shaken, it began to gulp down the stars around it, thereby revealing its presence.
That's the picture painted by new observations of HLX-1, the strongest candidate yet found for an apparently rare type of "intermediate-mass" black hole. This type of hole falls between stellar-mass black holes, which weigh up to a few tens of suns, and supermassive black holes, weighing millions or billions of suns.
It had not been clear whether middleweight black holes formed inside ancient star clusters or by repeated mergers of small galaxies. The new study not only suggests they grew by mergers, but it also hints they will keep merging. That could explain another mystery ? how supermassive black holes got so big.
Missing link
Stellar-mass black holes form from the collapse of massive stars
If they grow gradually, astronomers should see millions of middleweight black holes on their way to forming supermassive ones ? and they don't. That could mean either they're not there, or are invisible.
Luckily, a handful of middleweights have shown themselves via the light that matter emits just before it disappears down their maws. Indeed, HLX-1 is thought to be devouring a star, causing it to shine brightly in X-rays (HLX stands for "hyperluminous X-ray source"). Its copious X-ray output led to its discovery in 2009 on the edge of ESO?243-49, a giant spiral galaxy.
Finding it there raised other questions, though. Where did it come from? And why was it living in the galactic suburbs?
Keeping pace
Remember that middleweight black holes are thought to form in one of two ways: from merging stellar-mass black holes in a globular cluster
To judge between the two scenarios, Farrell and colleagues used the Hubble Space Telescope to observe HLX-1 in a range of wavelengths. They found that the black hole is surrounded by a cluster of stars, which are too small to be seen individually but together give a clear spectral signal.
The stars' colour suggests they are youngsters, no more than 200 million years old. That timing is suspicious ? ribbons of dust in the larger galaxy suggest it ate a dwarf galaxy some 200 million years ago, Farrell says. He thinks the collision stripped most of the dust and gas from the dwarf galaxy, but not all ? what was left was compressed around the dwarf's central black hole, triggering a new burst of star formation around HLX-1.
"This tells us that the idea of a dwarf galaxy hosting an intermediate mass black hole is viable," Farrell says. "We thought they should be out there. This is the first time we've seen one that fits."
Fade away
Fabien Gris? of the University of Iowa in Iowa City thinks the explanation is reasonable: "Galaxies, as we know them today, have been likely to accrete a certain number of dwarf galaxies in their history, so we know that this is not a rare process."
In that case, past is likely to prove prologue. "This explanation could also fit in the scenario where supermassive black holes were built with the merger of smaller, intermediate-mass, black holes," says Gris?. Eventually, HLX-1 may get sucked into the larger galaxy's centre, feeding its monstrous black hole.
However, Gris? says the case for this black hole's provenance is not yet closed. Another recent study suggests that while some of the optical light around HLX-1 is from stars, most is from gas falling into the black hole. In that case, the stars around it could be from an old globular cluster after all, since older stars are dimmer than younger ones.
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