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An international
team, including an astronomer from Imperial College London, discovered the
massive galaxy when trawling through data from the European Space
Agency’s Gaia satellite.
Despite its size, the galaxy has very few stars, challenging conventional
theories of galaxy formation.
“Antlia 2 might be hinting towards some new fundamental forces at work.”Dr Alex Geringer-Sameth

The galaxy,
named Antlia 2 (or
Ant 2), has avoided detection until now thanks to its extremely low density as
well as a perfectly chosen hiding place, behind the Milky Way’s bright central
disc. Ant 2 is known as a dwarf galaxy. As structures emerged in the early
Universe, dwarfs were the first galaxies to form, and so most of their stars
are now old, relatively low in terms of their mass, and poor in metals.
However, compared
to the other known dwarf galaxies orbiting the Milky Way, Ant 2 is immense. It
is as big as the Large Magellanic Cloud (LMC), a bright satellite galaxy that
is a third the size of the Milky Way itself.
New fundamental forces at work?
Ant 2 is also
unusual because of how little light it gives out: compared to the LMC, Ant 2 is
10,000 times fainter, meaning that it has about one 10,000th the number of
stars.
Current theories
of galaxy formation cannot fully explain how a galaxy so large has so few
stars. Previous research has suggested that some dwarfs could be inflated by
vigorous star formation, causing the galaxy to ‘puff up’, but the new research
shows that this process would have to be remarkably efficient to explain Ant 2.
Alternatively, the
properties of the elusive ‘dark matter’, thought to keep galaxies together,
might need to be re-thought. The currently favoured theory predicts that dark
matter packs tightly in the centres of galaxies, but a dark matter particle
which is less keen to cluster may be required.

Co-author Dr Alex
Geringer-Sameth, from the Department of Physics at Imperial, said:
“Antlia 2 might be hinting towards some new fundamental forces at work — in particular that dark matter might not behave as simply as has been assumed in the standard model of cosmology.”
Enough data to spot a ghost
The researchers
behind the current study – from Taiwan, the UK, the US, Australia and Germany –
searched the new Gaia data looking for Milky Way satellites by using RR Lyrae
stars. These stars are old and metal-poor, typical of those found in dwarf
galaxies.
The ESA’s Gaia
mission has produced the richest star catalogue to date, including
high-precision measurements of nearly 1.7 billion stars and revealing
previously unseen details of our home galaxy. Earlier this year, Gaia’s second
data release made new details of stars in the Milky Way available to scientists
worldwide.
Lead author
Gabriel Torrealba, from the Institute of Astronomy and Astrophysics at Academia
Sinica in Taipei, Taiwan, said:
“This is a ghost of a galaxy. Objects as diffuse as Ant 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data.”
The tip of the galactic iceberg
The team were able
to confirm that the ghostly object they spotted was real by checking with
colleagues at the Anglo-Australian Telescope (AAT) in Australia, which showed
that all the stars were moving together. The researchers were also able to
estimate the galaxy’s mass, which was much lower than expected for an object of
its size.
Dr Geringer-Sameth
said:
“Antlia 2 certainly gives a reason for optimism. One of the many great things about Gaia is that it surveys the entire sky. I love the idea that the next discovery may already be waiting in the public archive of Gaia data. We just have to learn how to spot it.”
Co-author Dr
Matthew Walker, from Carnegie Mellon University, added:
“We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one.”
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