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The ones

that got away

Our black hole collection is missing some
prize specimens. Now we might know where
they’ve been hiding, says Colin Stuart

ARMEZCUAspendsherdayshunting recently, gravitational waves emitted by
invisiblegame.Lumberinggiants, the collision of black holes have also been
Mimpossibletoseewiththenaked observed. One way or another, black hole
eye,expertlycamouflagedinthedarkness hunters have lined their walls with an
ofthenightsky.Sheknowsthey’reoutthere; impressive haul of these behemoths.
shehasseentheirfootprints,trackedtheir Yet for all their successes, there is still
spoor.Butforallthehoursshehasspentlying a trophy missing from their collection.
inwaitforblackholes,thereisonebreedshe For the most part, the black holes found
hasneverspotted.Onpaper,theyarefairly so far can be split into two camps. At the puny
unremarkable:averagesize,averagemass. end are stellar mass black holes, formed by
Theyaren’tsupposedtobeanybetterat the explosive deaths of massive stars and
hidingthantheothers,andtheyshould typically weighing less than 100 times the
existinroughlyequalnumbers.Butthey mass of our sun. At the other end are the
justaren’tthere. supermassive black holes that sit at the heart
Spotting these missing black holes won’t of galaxies. The monster at the centre of our
just fill a hole in her collection. It could shed Milky Way comes in at 4 million solar masses.
light on a central mystery of black hole Other galaxies have black holes running to
evolution: how small ones can get so big so billions of solar masses. But the in-betweeners,
quickly. What’s more, it could hold the key to intermediate mass black holes (IMBHs),
the unusual behaviour of some galaxies. It’s a are conspicuous by their absence.
cosmic conundrum and, according to Mezcua, This is a puzzling state of affairs.
an astrophysicist at the Institute of Space Supermassive black holes have to come
Sciences in Barcelona, one we might be on from somewhere, and the dominant theory
the verge of solving. suggests they form when smaller black holes
Black holes are not easy to study. They are so merge. But building a billion-solar-mass
massive and compact that their gravity sucks Goliath from millions of tiny Davids takes
in anything venturing too close. Even light time. A lot of time. So when astronomers
cannot escape their gravitational clutches, started observing fully formed supermassive
so they reflect nothing, rendering them all but black holes floating about in the first billion
invisible. That has forced astronomers to get years after the big bang, they knew something
creative with their searches. Stars accelerating was up. “There wasn’t time for these
around an invisible body are one giveaway of supermassive black holes to grow so big in RICHARD WILKINSON
a black hole, as are sudden bursts of X-rays such a young universe,” says Mezcua. Unless,
released by material falling to its doom. More that is, they were built from IMBHs. If the

32 | NewScientist | 20 January 2018