MAR MEZCUA spends her days hunting invisible game. Lumbering giants, impossible to see with the naked eye, expertly camouflaged in the darkness of the night sky. She knows they’re out there; she has seen their footprints, tracked their spoor. But for all the hours she has spent lying in wait for black holes, there is one breed she has never spotted. On paper, they are fairly unremarkable: average size, average mass. They aren’t supposed to be any better at hiding than the others, and they should exist in roughly equal numbers. But they just aren’t there.
Spotting these missing black holes won’t just fill a hole in her collection. It could shed light on a central mystery of black hole evolution: how small ones can get so big so quickly. What’s more, it could hold the key to the unusual behaviour of some galaxies. It’s a cosmic conundrum and, according to Mezcua, an astrophysicist at the Institute of Space Sciences in Barcelona, one we might be on the verge of solving.
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Black holes are not easy to study. They are so massive and compact that their gravity sucks in anything venturing too close. Even light cannot escape their gravitational clutches, so they reflect nothing, rendering them all but invisible. That has forced astronomers to get creative with their searches. Stars accelerating around an invisible body are one giveaway of a black hole, as are sudden bursts of X-rays released by material falling to its doom. More recently, gravitational waves emitted by the collision of black holes have also been observed. One way or another, black hole hunters have lined their walls with an impressive haul of these behemoths. Yet for all their successes, there is still a trophy missing from their collection.
For the most part, the black holes found so far can be split into two camps. At the puny end are stellar mass black holes, formed by the explosive deaths of massive stars and typically weighing less than 100 times the mass of our sun. At the other end are the supermassive black holes that sit at the heart of galaxies. The monster at the centre of our Milky Way comes in at 4 million solar masses. Other galaxies have black holes running to billions of solar masses. But the in-betweeners, intermediate mass black holes (IMBHs), are conspicuous by their absence.
This is a puzzling state of affairs. Supermassive black holes have to come from somewhere, and the dominant theory suggests they form when smaller black holes merge. But building a billion-solar-mass Goliath from millions of tiny Davids takes time. A lot of time. So when astronomers started observing fully formed supermassive black holes floating about in the first billion years after the big bang, they knew something was up. “There wasn’t time for these supermassive black holes to grow so big in such a young universe,” says Mezcua. Unless, that is, they were built from IMBHs. If the in-betweeners sprang into existence ready-made, either from the explosive deaths of gargantuan stars or from the direct collapse of clouds of gas, there would have been enough time for some to merge into supermassive monsters. But if that’s the case, there should be plenty of IMBHs left over that didn’t merge. “We should be able to observe them now in the local universe,” says Mezcua. Where are they all hiding?
Until recently, we thought we had it cracked. The answer seemed to lie in a series of unusually bright X-ray flashes that telescopes around the world had been detecting since the 1980s. As matter spiralling into black holes travels faster the closer it gets to destruction, friction with neighbouring material heats it until it glows in X-rays. The greater the mass of the central object, the faster the spiralling matter flies, the greater the friction and the brighter the X-rays. Less powerful X-ray blasts had already been used to pinpoint small black holes, but these new ones, more than a million times brighter than the sun, appeared too bright to be coming from black holes of only stellar mass size. The evidence seemed to point to IMBHs instead.
That picture continued to hold as data accumulated, but there was a nagging doubt. “There had been hints for some years that the spectra of these objects didn’t quite fit with IMBHs,” says Matteo Bachetti from the Cagliari Astronomical Observatory in Italy. Then in 2014 came a bombshell: one of the X-ray blasts was found to be pulsating. Then another. And another.
The discovery left astronomers reeling. “It was so shocking,” says Bachetti. The X-ray signatures were characteristic of pulsars, the dead cores of medium-sized stars rotating rapidly, spitting out radio waves at their poles like a cosmic lighthouse. But the pulses were more than 100 times brighter than a pulsar should be. In research published in August 2017, Grzegorz Wiktorowicz from the University of Warsaw in Poland suggested this apparent super-luminosity arises from the narrow beams the pulsar directs at us. In assuming that the compact object was sustaining that glow in all directions we had overestimated its mass. The implication is that “intermediate mass black holes are not needed to explain ultra-luminous X-ray blasts”, says Wiktorowicz.
Is there anything else we might have been missing? Last February, a team of astronomers led by Bülent Kiziltan at Harvard University announced a discovery in a dense group of ancient stars known as a globular cluster. The clusters are too old to spot a black hole in them by looking for swarms of stars or glowing accretion discs. “Radiation from the outside cocoon of stars blows away the accretion disc over time and any nearby stars have been gobbled up or ejected already,” says Kiziltan.
“We should be able to see these medium-sized black holes today. Where are they all hiding?”
Instead, he and his collaborators have come up with a method of detection based on the path pulsars trace through the sky, allowing them to measure very tiny variations in their accelerations. His team turned its attention to the cluster 47 Tucanae, known as 47 Tuc, visible in the southern hemisphere constellation of the Toucan. They found that some of 47 Tuc’s pulsars are being accelerated by an additional gravitational pull on top of that provided by the cluster’s stars. They put that down to a central black hole weighing between 1450 and 3800 solar masses, right in the middle of the range for IMBHs. “We believe we’ve finally found one,” Kiziltan says.
Despite this potential sighting, globular clusters are unlikely to play host to enough IMBHs to make up for the shortfall in them. “We are fairly certain that they do not reside in every globular cluster,” says Kiziltan. “There have to be very fine-tuned parameters in order to produce and maintain the black hole.” As the densest and most massive of the globular clusters surrounding the Milky Way, 47 Tuc has all the right attributes, but it seems to be the exception and not the rule. Kiziltan may have bagged an important specimen, but the trophy cabinet remains largely empty.
Happy hunting ground
But all is not lost for the IMBH hunters. Help filling the shelves may come from an unlikely source – other missing cosmic entities. While some astronomers have been scouring the skies for IMBHs, others have been looking for missing dwarf galaxies. These dwarfs, as their name suggests, aren’t huge and are often found orbiting larger galaxies such as our own Milky Way. The trouble is, says Joseph Silk, an astrophysicist at the University of Oxford, “we don’t observe anywhere near enough of them”.
In our standard picture of cosmology, galaxies and galaxy clusters are permeated by dark matter, a sluggish invisible entity whose gravitational attraction holds structures together. When astronomers run computer simulations of galaxy formation in the early universe, they end up with a lot of dwarf galaxies that didn’t merge. Yet we see far fewer of them in the real universe. There’s another problem, too, says Silk: stars in the centre of the dwarfs we do see are not orbiting fast enough. The standard theory predicts that there should be a dense mass of dark matter at the heart of a dwarf galaxy known as a “cusp”. Its gravity should make stars whizz around at a much greater lick than we observe.
“One solution has a pleasing logic – as galaxies combine, their black holes do too”
These issues have led some astronomers to argue that we need to change the way we think about dark matter. Conventionally, cosmologists refer to dark matter as “cold” – meaning bulky and slow-moving like molecules in a cool gas. Yet if the mass of dark matter particles is lower than we had counted on, they’d be zippier and less inclined to clump together. “It would have acted as an egg-beater in the early universe, mixing things up and erasing smaller structures,” says Marla Geha at Yale University. Perhaps that’s why we don’t see as many dwarf galaxies as the standard theory predicts. Such zippy, “warm” dark matter would also be too restless to clump together at the centre of dwarf galaxies, leading to a smaller core rather than a cusp, as well as stars that orbit more slowly.
It’s a viable explanation, but Silk prefers a less radical approach. “I find it strange to invent new physics to solve a problem that may well be solved by known physics,” he says. In a paper published last April, he argues that there is a simpler solution to the mysteries surrounding dwarf galaxies: most have central IMBHs.
On the face of it, Silk’s proposal has a pleasing logic. Just as large galaxies have large black holes at their centre, smaller galaxies would have ones tailored to match. And as the smaller galaxies drew together to form larger structures, their central black holes would have combined as well.
Vanishing double act
The pairing of medium-sized black holes with small galaxies makes sense on a deeper level too. Early in a dwarf galaxy’s life, the IMBH would have been fed by lots of gas, creating huge outward eruptions that destroyed much of the galaxy. “You end up with lots of dwarf galaxies, but they are almost all little,” says Silk. These smaller dwarf galaxies would not be as bright, perhaps explaining why we’ve struggled to see them so far. This idea gained backing in November when a team of astronomers led by Stacy Kim from Ohio State University used data from the Sloan Digital Sky Survey to estimate how many faint dwarf galaxies we might yet find around our own Milky Way. The answer was largely in agreement with the predictions of a cosmology based on cold dark matter.
The cusp problem can also be explained by an early, active IMBH, because the X-ray blasts it emitted would have blown away much of the central accumulation of dark matter. Not everyone agrees with that interpretation, however. “An IMBH will only affect a very tiny region, probably much much smaller than the kind of scales on which people are looking for cores or cusps,” says Andrew Cooper, a dwarf galaxy researcher at Durham University, UK.
Circumstantial evidence for IMBHs in dwarf galaxies has been around since the late 1980s. So, if Silk is right, why haven’t we found more concrete evidence of their existence in the intervening decades? Last March, a study based on survey data from the NuSTAR space telescope provided a possible answer. Analysis of 40 months of observations suggests that low-mass galaxies absorb a lot of X-rays, so almost half of all active black holes in the centre of low-mass galaxies would have had their explosive emissions absorbed before they reach us. There is another reason why the rest may have remained largely under the radar, too. By gorging on gas early in their lives, they produced outbursts that blew away much of their future food supply. “For every active black hole there should be 10 passive ones,” says Silk. “You have to catch the black hole at the right moment to see it in X-rays.” Even then, the NuSTAR survey revealing X-ray absorption suggests that the signal reaching us will be faint. That pushes current X-ray telescopes such as the orbiting Chandra observatory to their limit. “It takes a special effort on the part of X-ray astronomers,” says Silk. “That’s why this area was overlooked until a year or two ago.”
Mezcua agrees that dwarf galaxies are the most promising places to look for the missing IMBHs, though what is less certain is how they got there. There are two main options: either they formed when the first massive stars collapsed, or else they were fashioned when giant gas clouds buckled under their own weight. The second mechanism would have created IMBHs on average 10 times more massive than the first, in keeping with the candidates found so far, but the sensitivity of the process means they would be relatively rare. According to Mezcua, “if the black holes were formed from stars then 90 per cent of local dwarf galaxies should have them.” That drops to 50 per cent if they came from gas clouds. Silk is more optimistic. “Both scenarios can give IMBHs in all dwarf galaxies,” he says.
There is a way we can settle the debate once and for all: looking out for gravitational waves. So far, the handful of gravitational wave events detected have come from colliding neutron stars or stellar mass black holes. “The next step is expected to be the detection of colliding IMBHs,” Mezcua says. If and when that starts happening, she may finally be able to complete her collection.
This article appeared in print under the headline “The ones that got away”
