Galaxy ESO 243-49 is 290 million light-years away and home to HLX-1 (circled), the leading intermediate mass black hole candidate. ( NASA, ESA, S. Farrell)
Galaxy ESO 243-49 is 290 million light-years away and home to HLX-1 (circled), the leading intermediate mass black hole candidate. ( NASA, ESA, S. Farrell)

The Mystery of the Missing Black Holes

In the census of black holes, there are basically two populations: The big, supermassive bruisers that churn away in the hearts of large galaxies, and the smaller, comparatively runty black holes that form when massive stars collapse and die.

But what about the medium-size black holes – the middleweights, the golden retrievers, the four-door sedans – that are neither galactic drain nor single stellar corpse?

According to some astronomers, these intermediate black holes, with masses equivalent to anywhere between 100 and 100,000 suns, should be everywhere. After all, those million-solar-mass galactic drains aren’t just born that way.

Trouble is, the middleweights are mostly missing. Decades of searching have only yielded a handful of strong candidates. Even this week’s episode of Cosmos: A Spacetime Odyssey, which dealt at length with black holes, didn’t mention the missing middleweights. But whether astronomers are having a hard time finding these intermediate mass black holes because they’re tricky to find, or just really, really hard to make is an open question.

“Black holes in general are hard to find because by definition, they don’t radiate any electromagnetic radiation,” says Sydney-based astrophysicist Sean Farrell.

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Artist’s conception of a lonely black hole, floating near a star cluster on the outskirts of the Milky Way. (David A. Aguilar, CfA)

In 2008, graduate student Joana Rodrigues spotted what’s still the leading intermediate mass black hole candidate. Rodrigues, then at the Universite de Toulouse, was working with Farrell and his colleagues; she was looking for lonely neutron stars and other objects that shone brightly in x-rays, using the European Space Agency’s XMM-Newton space telescope.

Rodrigues saw something unexpected: An object that glowed extremely brightly in x-rays – what astronomers call an ultra-luminous x-ray source, or ULX.  Hovering 290 million light-years away in the halo of a galaxy known as ESO 243-49, the object was perplexing for several reasons.

First, this wasn’t just any old super-bright thing. It was ridiculously bright – more than 10 times brighter than any known ULX, and at least 100 million times brighter than the sun. That incredible brightness suggested something very energetic and active was taking place. And, the object, now called HLX-1 (for hyper-luminous x-ray source), wasn’t in the right place. Rather than being at the heart of its galaxy, it was off in the galactic periphery. Lastly, there seemed to be a disk of hot, turbulent gas swirling around it.

At first, Farrell says, he was pretty sure they’d gotten the object’s location wrong. All signs pointed to HLX-1 being a massive black hole in the process of yanking material from some nearby stars – the kind of thing that should be in the heart of a galaxy, rather than on the outside.

But follow-up observations suggested that HLX-1 really was on the fringe, and that the superhot gas likely came from a star being ripped apart by a black hole with several hundred solar masses.

In other words, the team seemed to have found an intermediate mass black hole.

Now, nearly six years later, HLX-1’s story has gotten even weirder. New measurements suggest it’s probably around 10,000 solar masses – still within the intermediate range. And in addition to that gassy disk, Hubble space telescope observations point to a dense population of young blue stars clustered around the black hole.

“The location of HLX-1 out in the halo of ESO 243-49 is a very unusual place to find young stars,” Farrell says. “Something would have had to trigger the formation of these stars in the recent past.”

That something might be the recent collision and merger of a dwarf galaxy with ESO 243-49 – a dwarf galaxy whose stars were absorbed by the larger galaxy and whose heart was punted into space. That heart is HLX-1. It simultaneously clings to some of its original stars and also supports a younger population formed during the galactic merger.

“Mergers of dwarf galaxies with large galaxies like the Milky Way are really common,” Farrell says. “So if dwarfs contain central black holes, then such black holes should be deposited in the halos of large galaxies.”

At a meeting in the Netherlands this week, astronomer Roberto Soria of Australia’s Curtin University ranked the existing IMBH candidates. At the top of his list? HLX-1.

But even though HLX-1 is the strongest IMBH candidate, it isn’t alone. Another candidate, M82 X-1, is nipping at its heels (M82 X-1 was second on Soria’s list). In 2006, a team led by astrophysicist Philip Kaaret of the University of Iowa spotted M82 X-1, about 11 million light-years away.

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M82, a galaxy 11 million light-years away, hosts another intermediate mass black hole candidate. (NASA/ESA/Hubble Heritage Team)

If you look toward the Big Dipper’s ladle, on a dark night with good binoculars you can see M82, the candidate’s host galaxy (this is the same galaxy in which a type 1a supernova exploded earlier this year). It appeared to Kaaret and his colleague Hua Feng as though the object they’d found – also a ULX – measured between 200 and 800 solar masses, and was snacking on a nearby red giant star.

More recently, teams have found intermediate mass black hole candidates in an irregular dwarf galaxy called I Zwicky 18 and on the fringes of the Circinus spiral galaxy (though that one – at 90 solar masses – is a little on the light side to be a bona fide candidate, says its discoverer, Dominic Walton of Caltech).

The dearth of candidates begs the question of whether these medium black holes are really as numerous as expected. Some astronomers, like Kaaret, suggest maybe not.

“I think they are so hard to find because they are so hard to form,” he says. “Most ‘stellar mass’ black holes form in the collapse of a single star.  It looks like one can get up to masses of about 50 or 80 – but less than 100 – solar masses this way. Intermediate mass black holes need a different mechanism.”

Others scientists, like Farrell, think these black holes are just hard to find.

Because black holes don’t emit any electromagnetic radiation, they’re more or less invisible when thrust upon the backdrop of dark, empty space. So, astronomers searching the sky for these cosmic objects have to get kind of lucky. They need to be looking in the right place, with the right instrument. Unless you spy something like a telltale jet, or accretion disk, or cluster of strange young stars, it’s hard to know that you might be looking at a black hole.

Perhaps galactic halos are the place to look for this missing class of black hole, Farrell says.

If intermediate mass black holes do form the hearts of dwarf galaxies, and if dwarf galaxies frequently merge with one another and with larger galaxies, then there could be many dark, punted hearts floating on the peripheries of large galaxies. Just like HLX-1.

“I think the likelihood that there are potentially hundreds of quiet intermediate mass black holes floating around in galaxy halos is probably pretty high,” Farrell says.