Astronomers Have Spotted A Mysterious "Ghost Galaxy" Next To The Milky Way

The Milky Way is not alone in our corner of the Universe. It is surrounded by dozens of small companion galaxies, some orbiting very close and some further away. Researchers have now announced the discovery of a new dwarf galaxy and it doesn’t look quite how we would expect.

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The object is called Antlia 2, or Ant 2, and it doesn’t seem to be quite there. Let us explain: It is massive, as large as one-third of the Milky Way, but extremely faint. It is comparable in size to the Large Magellanic Cloud (LMC), one of our two galactic companions visible to the naked eye, but it's 10,000 times fainter. Basically, it is either far too large for its luminosity or far too dim for its size. A paper reporting the discovery is available on ArXiv.

This is a ghost of a galaxy,” Gabriel Torrealba, the paper’s lead author from the University of Cambridge, said in a statement. “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.

Gaia is the flagship European mission that is mapping the position of billions of stars from the Milky Way and beyond. Its high precision measurements have allowed researchers to track the motion of millions of stars, which is how they discovered a group of distant stars moving together. After this first analysis, the researchers used the Anglo-Australian Telescope to study 100 more stars, which allowed them to estimate the distance and mass of Ant 2.

It is located 130,000 light-years from the Milky Way and is over 13 million times heavier than the Sun. For a galaxy, though, that is definitely on the light side. The LMC, for example, is almost 1,000 times heavier.

The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the Galactic tides of the Milky Way,” said co-author Sergey Koposov from Carnegie Mellon University. “What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow.

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One possible explanation is that Ant 2 formed in a region of space where dark matter was not very dense and due to internal processes like supernova explosions and stellar winds, gas spreads out. Over time stars formed at these edges, and the small galaxy got its current size. But this explanation requires either very efficient explosions or dark matter behaving differently to how we expect. It is unclear at the moment what the dominant factor is.

Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball,” said co-author Matthew Walker. “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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