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Enormous “Ghost” Galaxy Spotted Hiding Next To The Milky Way

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

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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.

Illustration of the galaxies and the Gaia satellite's position in the Milky Way

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.

Image result for Antlia 2 galaxy

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.”