The discovery of two massive
holes punched through a stream of stars could help answer questions
about the nature of dark matter, the mysterious substance holding
galaxies together. Researchers have detected two massive holes which
have been ‘punched’ through a stream of stars just outside the Milky
Way, and found that they were likely caused by clumps of dark matter,
the invisible substance which holds galaxies together and makes up a
quarter of all matter and energy in the universe.
The scientists, from the University of Cambridge, found the holes by
studying the distribution of stars in the Milky Way. While the clumps
of dark matter that likely made the holes are gigantic in comparison to
our Solar System – with a mass between one million and 100 million times
that of the Sun – they are actually the tiniest clumps of dark matter
detected to date.
“While we do not yet understand what dark matter is formed of, we
know that it is everywhere,” said Dr Denis Erkal from Cambridge’s
Institute of Astronomy, the paper’s lead author. “It permeates the
universe and acts as scaffolding around which astrophysical objects made
of ordinary matter – such as galaxies – are assembled.”
The results, which have been submitted to the Monthly Notices of the
Royal Astronomical Society, could help researchers understand the
properties of dark matter, by inferring what type of particle this
mysterious substance could be made of. According to their calculations
and simulations, dark matter is likely made up of particles more massive
and more sluggish than previously thought, although such a particle has
yet to be discovered.
Current theory on how the universe was formed predicts that many of
these dark matter building blocks have been left unused, and there are
possibly tens of thousands of small clumps of dark matter swarming in
and around the Milky Way. These small clumps, known as dark matter
sub-haloes, are completely dark, and don’t contain any stars, gas or
dust.
Dark matter cannot be directly measured, and so its existence is
usually inferred by the gravitational pull it exerts on other objects,
such as by observing the movement of stars in a galaxy. But since
sub-haloes don’t contain any ordinary matter, researchers need to
develop alternative techniques in order to observe them.
The technique the Cambridge researchers developed was to essentially
look for giant holes punched through a stream of stars. These streams
are the remnants of small satellites, either dwarf galaxies or globular
clusters, which were once in orbit around our own galaxy, but the strong
tidal forces of the Milky Way have torn them apart. The remnants of
these former satellites are often stretched out into long and narrow
tails of stars, known as stellar streams.
“Stellar streams are actually simple and fragile structures,” said
co-author Dr Sergey Koposov. “The stars in a stellar stream closely
follow one another since their orbits all started from the same place.
But they don’t actually feel each other’s presence, and so the apparent
coherence of the stream can be fractured if a massive body passes
nearby. If a dark matter sub-halo passes through a stellar stream, the
result will be a gap in the stream which is proportional to the mass of
the body that created it.”
The researchers used data from the stellar streams in the Palomar 5
globular cluster to look for evidence of a sub-halo fly-by. Using a new
modelling technique, they were able to observe the stream with greater
precision than ever before. What they found was a pair of wrinkled tidal
tails, with two gaps of different widths.
By running thousands of computer simulations, the researchers
determined that the gaps were consistent with a fly-by of a dark matter
sub-halo. If confirmed, these would be the smallest dark matter clumps
detected to date.
“If dark matter can exist in clumps smaller than the smallest dwarf
galaxy, then it also tells us something about the nature of the
particles which dark matter is made of – namely that it must be made of
very massive particles,” said co-author Dr Vasily Belokurov. “This would
be a breakthrough in our understanding of dark matter.”
The reason that researchers can make this connection is that the mass
of the smallest clump of dark matter is closely linked to the mass of
the yet unknown particle that dark matter is composed of. More
precisely, the smaller the clumps of dark matter, the higher the mass of
the particle.
Since we do not yet know what dark matter is made of, the simplest
way to characterise the particles is to assign them a particular energy
or mass. If the particles are very light, then they can move and
disperse into very large clumps. But if the particles are very massive,
then they can’t move very fast, causing them to condense – in the first
instance – into very small clumps.
“Mass is related to how fast these particles can move, and how fast
they can move tells you about their size,” said Belokurov. “So that’s
why it’s so interesting to detect very small clumps of dark matter,
because it tells you that the dark matter particle itself must be very
massive.”
“If our technique works as predicted, in the near future we will be
able to use it to discover even smaller clumps of dark matter,” said
Erkal. “It’s like putting dark matter goggles on and seeing thousands of
dark clumps each more massive than a million suns whizzing around.”
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