An Entire Swarm of Black Holes Has Been Caught Moving Through The Milky Way


A fluffy
cluster of stars spilling across the sky may have a secret hidden in its heart:
a swarm of over 100 stellar-mass black holes.


If this
finding can be validated, it will explain how the cluster came to be the way it
is - with its stars spaced light-years apart, smearing out into a stellar
stream stretching across 30,000 light-years.


The star
cluster in question is called Palomar 5, located around 80,000 light-years
away. Such globular clusters are often considered 'fossils' of the early
Universe. They're very dense and spherical, typically containing roughly
100,000 to 1 million very old stars; some, like NGC 6397, are nearly as old as
the Universe itself.


In any
globular cluster, all its stars formed at the same time, from the same cloud of
gas. The Milky Way has around 150 known globular clusters; these objects are
excellent tools for studying, for example, the history of the Universe, or the
dark matter content of the galaxies they orbit.


But there's
another type of star group that is gaining more attention - tidal streams, long
rivers of stars that stretch across the sky. Previously, these had been difficult
to identify, but with the Gaia space observatory working to map the Milky Way
with high precision in three dimensions, more of these streams have been
brought to light.


"We do
not know how these streams form, but one idea is that they are disrupted star
clusters," explained astrophysicist Mark Gieles of the University of
Barcelona in Spain.


none of the recently discovered streams have a star cluster associated with
them, hence we can not be sure. So, to understand how these streams formed, we
need to study one with a stellar system associated with it. Palomar 5 is the
only case, making it a Rosetta Stone for understanding stream formation and
that is why we studied it in detail."


Palomar 5
appears unique in that it has both a very wide, loose distribution of stars and
a long tidal stream, spanning more than 20 degrees of the sky, so Gieles and
his team homed in on it.


The team
used detailed N-body simulations to recreate the orbits and evolutions of each
star in the cluster, to see how they could have ended up where they are today.


Since recent
evidence suggests that populations of black holes could exist in the central
regions of globular clusters, and since gravitational interactions with black
holes are known to send stars careening away, the scientists included black
holes in some of their simulations.


results showed that a population of stellar-mass black holes within Palomar 5
could have resulted in the configuration we see today. Orbital interactions
would have slingshot the stars out of the cluster and into the tidal stream,
but only with a significantly higher number of black holes than predicted.


The stars
escaping the cluster more efficiently and readily than black holes would have
altered the proportion of black holes, bumping it up quite a bit.


number of black holes is roughly three times larger than expected from the
number of stars in the cluster, and it means that more than 20 percent of the
total cluster mass is made up of black holes," Gieles said.

"They each
have a mass of about 20 times the mass of the Sun, and they formed in supernova
explosions at the end of the lives of massive stars, when the cluster was still
very young."


In around a
billion years, the team's simulations showed, the cluster will dissolve
completely. Just before this happens, what remains of the cluster will consist
entirely of black holes, orbiting the galactic center. This suggests that
Palomar 5 is not unique, after all - it will dissolve completely into a stellar
stream, just like others that we have discovered.


It also
suggests that other globular clusters will likely share the same fate,
eventually. And it offers confirmation that globular clusters may be excellent
places to look for black holes that will eventually collide, as well as the
elusive class of middleweight black holes, between stellar mass lightweights
and supermassive heavyweights.


"It is
believed that a large fraction of binary black hole mergers form in star
clusters," said astrophysicist Fabio Antonini of Cardiff University in the

"A big
unknown in this scenario is how many black holes there are in clusters, which
is hard to constrain observationally because we can not see black holes. Our
method gives us a way to learn how many black holes there are in a star cluster
by looking at the stars they eject."


The research
has been published in Nature Astronomy.