How did supermassive black holes form? Two studies discovered dozens of middling-mass black holes in dwarf galaxies to fuel an ongoing debate.

Heavyweight black holes are a surprisingly common feature of the universe, lurking or blazing in galaxies near and far. But a question has long troubled astronomers: Where did these so-called supermassive black holes — typically ranging in mass from millions to billions of Suns — come from? Two studies using X-ray observations give two different answers.

Past & Present

When the universe was less than a billion years old, galaxies were already hosting supermassive black holes, visible by the glowing gas they gobbled down at rates of dozens of Suns per year. Even when gorging at that rate, though, the typical way we know of to make a black hole — the collapse of a dying star — wouldn’t suffice to create such beasts.

There are two solutions currently at the forefront of solving this dilemma: One idea is that the first generation of stars that gave birth to black holes were far more massive than the modern version, typically containing 100 Suns’ worth of mass. These 100-solar-mass black holes would then have to feed at tremendous rates pretty much all the time to grow to supermassive size by the time the universe reaches 1 billion years old.

An alternative idea is that the first black holes formed when a gas cloud directly collapsed into something between up to a thousand times the mass of the Sun. This scenario needs a pretty special situation to grow such massive black holes, to keep all the gas together as it’s collapsing without having it fragment into stars. (See Camille Carlisle's feature, "The First Black Holes," in the January 2017 issue of Sky & Telescope for more detail on these scenarios.)

Current technology won't let us actually test either scenario, so astronomers take a roundabout route to look for the answer. If the supermassive black holes that inhabit large galaxies start their lives as stellar-mass black holes, then smaller, “intermediate-mass” black holes should be common in smaller galaxies. The direct-collapse scenario instead predicts that such middling black holes would be rare. So all astronomers have to do is tally up the black holes in dwarf galaxies and they have their answer — but that's not as easy as it sounds.

Tallying the Middleweight Division

Mar Mezcua (Institute of Space Science, Spain) and colleagues pored through X-ray images collected as part of the Chandra COSMOS Legacy Survey, which also includes visible-light and infrared data. They find 40 dwarf galaxies hosting actively feeding, middleweight black holes with masses between 10,000 and 100,000 Suns. The team uses the fraction of “active” black holes — roughly 10% — as a proxy for the total number of black holes in dwarf galaxies. The low fraction, they argue in the August 1st Monthly Notices of the Royal Astronomical Society, favors the direct-collapse model.

Another point in favor of the direct-collapse scenario is that it predicts that smaller galaxies should be less likely to host black holes, and that is exactly what Mezcua and colleagues find. But the fraction of active black holes that the team calculates is in reality a lower limit on the true number of black holes. It could be that huge numbers of fasting black holes, silent in X-rays, lurk undetected in other dwarf galaxies.

Adding to the debate are new data from Igor Chilingarian (Smithsonian Astrophysical Observatory) and colleagues, who data-mined sky surveys to find a sample of 305 middling-mass black hole candidates, published August 6th in the Astrophysical Journal. They confirm 10 of these candidates with new and archived X-ray observations from Chandra, XMM-Newton, and Swift, estimated to have masses between 30,000 and 200,000 Suns.

Chilingarian argues that the mere existence of intermediate-mass black holes on the lower end of this mass spectrum favors the stellar-seed scenario, as the direct-collapse scenario ought to produce only black holes with more than 100,000 Suns’ worth of mass. But Mezcua disagrees, arguing that the direct-collapse models are flexible enough to allow for smaller masses.

For now, the contest ends in a tie, and that’ll probably remain the case until astronomers can figure out a way to find dormant black holes in dwarf galaxies.

“The methods used to measure black hole masses or to infer the presence of a black hole in massive galaxies with dormant black holes cannot be applied to dwarf galaxies, because these are small and faint compared to massive elliptical galaxies,” Mezcua says. “So if detecting active black holes in dwarf galaxies is difficult, detecting the dormant ones is even more challenging.”

Amy Reines, whose 2013 study also favored the direct-collapse model with the same caveats as the study from Mezcua's team, agrees. “I don't think there is conclusive evidence one way or the other,” says Amy Reines (Montana State University). “More work needs to be done.”