Page 36 - New Scientist

Then another. And another. IMBHs to make up for the shortfall in them.
The discovery left astronomers reeling. “We are fairly certain that they do not reside
“It was so shocking,“ says Bachetti. The X-ray in every globular cluster,”says Kiziltan.“There
signatures were characteristic of pulsars, the have to be very fine-tuned parameters in order
dead cores of medium-sized stars rotating to produce and maintain the black hole.”
rapidly, spitting out radio waves at their poles As the densest and most massive of the
like a cosmic lighthouse. But the pulses were globular clusters surrounding the Milky Way,
more than 100 times brighter than a pulsar 47 Tuc has all the right attributes, but it seems
should be. In research published in August to be the exception and not the rule. Kiziltan
2017, Grzegorz Wiktorowicz from the may have bagged an important specimen,
University of Warsaw in Poland suggested but the trophy cabinet remains largely empty.
this apparent super-luminosity arises from
the narrow beams the pulsar directs at us.
In assuming that the compact object was Happy hunting ground
sustaining that glow in all directions we had But all is not lost for the IMBH hunters. Help
overestimated its mass. The implication is filling the shelves may come from an unlikely
that“intermediate mass black holes are not source – other missing cosmic entities. While
needed to explain ultra-luminous X-ray some astronomers have been scouring the
blasts”, says Wiktorowicz. skies for IMBHs, others have been looking
Is there anything else we might have been for missing dwarf galaxies. These dwarfs, as
missing? Last February, a team of astronomers their name suggests, aren’t huge and are often
led by Bülent Kiziltan at Harvard University found orbiting larger galaxies such as our own
announced a discovery in a dense group of Milky Way. The trouble is, says Joseph Silk,
ancient stars known as a globular cluster. The an astrophysicist at the University of Oxford,
clusters are too old to spot a black hole in them “we don’t observe anywhere near enough
by looking for swarms of stars or glowing of them”.
accretion discs.“Radiation from the outside In our standard picture of cosmology,
cocoon of stars blows away the accretion disc galaxies and galaxy clusters are permeated

“We should be able to see these NASA/ESA/HUBBLE HERITAGE (STSCI/AURA)-ESA/HUBBLE COLLABORATION

medium-sized black holes today.

Where are they all hiding?”

over time and any nearby stars have been by dark matter, a sluggish invisible entity Black holes hiding at
gobbled up or ejected already,”says Kiziltan. whose gravitational attraction holds the heart of dwarf
Instead, he and his collaborators have come structures together. When astronomers run galaxies could be
up with a method of detection based on the computer simulations of galaxy formation in undetectable on Earth
path pulsars trace through the sky, allowing the early universe, they end up with a lot of
them to measure very tiny variations in dwarf galaxies that didn’t merge. Yet we see
their accelerations. His team turned its far fewer of them in the real universe. There’s
attention to the cluster 47 Tucanae, known another problem, too, says Silk: stars in the
as 47 Tuc, visible in the southern hemisphere centre of the dwarfs we do see are not orbiting
constellation of the Toucan. They found that fast enough. The standard theory predicts that
some of 47 Tuc’s pulsars are being accelerated there should be a dense mass of dark matter at
by an additional gravitational pull on top of the heart of a dwarf galaxy known as a“cusp”.
that provided by the cluster’s stars. They put Its gravity should make stars whizz around at
that down to a central black hole weighing a much greater lick than we observe.
between 1450 and 3800 solar masses, right in These issues have led some astronomers
the middle of the range for IMBHs.“We believe to argue that we need to change the way we
we’ve finally found one,”Kiziltan says. think about dark matter. Conventionally,
Despite this potential sighting, globular cosmologists refer to dark matter as“cold”–
clusters are unlikely to play host to enough meaning bulky and slow-moving like

34 | NewScientist | 20 January 2018