Astronomers have discovered an enormous ‘ghost' galaxy in the outskirts of the Milky Way. Dubbed Antlia 2 (or Ant 2), the galaxy is remarkably faint but is located only about 130,000 light-years away from our galaxy.
Astronomers, including from the University of Cambridge, spotted Antlia 2 using the data collected by the European Space Agency's Gaia satellite. This faint galaxy was found hiding in the region behind the Milky Way's disc - probably the reason why it could not be spotted by astronomers up until now.
According to astronomers, Antlia 2 is a dwarf galaxy, and was formed in the early universe. Compared to the other dwarf satellites of the Milky Way, it is huge in size, although only about one-third of the size of the Milky Way. It is as big as the Large Magellanic Cloud (LMC).
However, despite its size, Antlia 2 gives out little light. It is nearly 10,000 times dimmer than the LMC. It is also characterised by low mass, meaning it is also low in density.
"This is a ghost of a galaxy," said lead author Gabriel Torrealba.
"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."
Antlia 2 was spotted when astronomers studied Gaia's data in a search of Milky Way satellites. They started their investigation by first looking for RR Lyrae stars - old stars with low metal content commonly found in dwarf galaxies.
The team was able to spot a group of RR Lyrae stars above the galactic disc. When they observed these stars closely, they noticed a new object, a ghost galaxy. Astronomers then used the Anglo-Australian Telescope in Australia for further investigation. The team had a limited time window to observe the galaxy due to motion of the Earth.
They measured the spectra of about 100 red giant stars to confirm that the ghost galaxy they had discovered was real.
According to astronomers, Antlia 2 always stays at least 40 kiloparsecs (about 130,000 light-years) away from the Milky Way. The reason behind Antlia 2 having too little mass is that the galaxy is "being taken apart by the Galactic tides of the Milky Way," astronomers believe.
"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," said co-author Sergey Koposov from Carnegie Mellon University.
They are yet to explain how Antlia 2 became so much extended in size. Solving the puzzle would shed some new light on how the first structures in the early universe were formed.
The team is currently busy searching for similar ghost dwarf galaxies in the universe.
Image credit: V. Belokurov, based on the images by Marcus and Gail Davies and Robert Gendler via the University of Cambridge
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