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“One solution has a pleasing

logic – as galaxies combine,

their black holes do too”

surrounding dwarf galaxies: most have so almost half of all active black holes in the
central IMBHs. centre of low-mass galaxies would have had
On the face of it, Silk’s proposal has a their explosive emissions absorbed before
pleasing logic. Just as large galaxies have large they reach us. There is another reason why
black holes at their centre, smaller galaxies the rest may have remained largely under
would have ones tailored to match. And as the the radar, too. By gorging on gas early in their
smaller galaxies drew together to form larger lives, they produced outbursts that blew away
structures, their central black holes would much of their future food supply. “For every
have combined as well. 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.”
Vanishing double act
Even then, the NuSTAR survey revealing
The pairing of medium-sized black holes with X-ray absorption suggests that the signal
small galaxies makes sense on a deeper level reaching us will be faint. That pushes current
too. Early in a dwarf galaxy’s life, the IMBH X-ray telescopes such as the orbiting Chandra
would have been fed by lots of gas, creating observatory to their limit. “It takes a special
huge outward eruptions that destroyed much effort on the part of X-ray astronomers,”
of the galaxy. “You end up with lots of dwarf says Silk. “That’s why this area was overlooked
galaxies, but they are almost all little,”says until a year or two ago.”
Silk. These smaller dwarf galaxies would Mezcua agrees that dwarf galaxies are the
not be as bright, perhaps explaining why most promising places to look for the missing
we’ve struggled to see them so far. This idea IMBHs, though what is less certain is how they
gained backing in November when a team of got there. There are two main options: either
astronomers led by Stacy Kim from Ohio State they formed when the first massive stars
University used data from the Sloan Digital collapsed, or else they were fashioned when
Sky Survey to estimate how many faint dwarf giant gas clouds buckled under their own
galaxies we might yet find around our own weight. The second mechanism would have
Milky Way. The answer was largely in created IMBHs on average 10 times more
agreement with the predictions of a massive than the first, in keeping with the
cosmology based on cold dark matter. candidates found so far, but the sensitivity of
molecules in a cool gas. Yet if the mass of dark The cusp problem can also be explained by the process means they would be relatively
matter particles is lower than we had counted an early, active IMBH, because the X-ray blasts rare. According to Mezcua, “if the black holes
on, they’d be zippier and less inclined to clump it emitted would have blown away much of were formed from stars then 90 per cent of
together. “It would have acted as an egg-beater the central accumulation of dark matter. local dwarf galaxies should have them.” That
in the early universe, mixing things up and Not everyone agrees with that interpretation, drops to 50 per cent if they came from gas
erasing smaller structures,” says Marla Geha however. “An IMBH will only affect a very tiny clouds. Silk is more optimistic. “Both scenarios
at Yale University. Perhaps that’s why we don’t region, probably much much smaller than the can give IMBHs in all dwarf galaxies,” he says.
see as many dwarf galaxies as the standard kind of scales on which people are looking for There is a way we can settle the debate once
theory predicts. Such zippy, “warm” dark cores or cusps,” says Andrew Cooper, a dwarf and for all: looking out for gravitational waves.
matter would also be too restless to clump galaxy researcher at Durham University, UK. So far, the handful of gravitational wave
together at the centre of dwarf galaxies, Circumstantial evidence for IMBHs in dwarf events detected have come from colliding
leading to a smaller core rather than a cusp, galaxies has been around since the late 1980s. neutrons stars or stellar mass black holes.
as well as stars that orbit more slowly. So, if Silk is right, why haven’t we found more “The next step is expected to be the detection
It’s a viable explanation, but Silk prefers concrete evidence of their existence in the of colliding IMBHs,” Mezcua says. If and when
a less radical approach. “I find it strange to intervening decades? Last March, a study that starts happening, she may finally be able
invent new physics to solve a problem that based on survey data from the NuSTAR space to complete her collection. Q
may well be solved by known physics,” he says. telescope provided a possible answer. Analysis
In a paper published last April, he argues that of 40 months of observations suggests that Colin Stuart (@skyponderer) is an astronomy writer
there is a simpler solution to the mysteries low-mass galaxies absorb a lot of X-rays, based in London

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