Astronomers using data from ESA’s Gaia satellite have discovered a new dwarf satellite galaxy of the Milky Way. The dwarf galaxy, named Antlia 2, is located roughly 130,000 light-years from Earth in the constellation Antlia. It 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 disk.
The Large Magellanic Cloud, the Milky Way Galaxy and Antlia 2 (from left to right). Image credit: V. Belokurov / Marcus and Gail Davies / Robert Gendler.
As structures emerged in the early Universe, dwarf galaxies were the first galaxies to form, and so most of their stars are old, low-mass and metal-poor.
But compared to the other known Milky-Way dwarf satellites, Antlia 2 is immense: it is as big as the Large Magellanic Cloud, and a third the size of the Milky Way itself.
What makes the newly-discovered galaxy even more unusual is how little light it gives out.
Compared to the Large Magellanic Cloud, Antlia 2 is 10,000 times fainter. In other words, it is either far too large for its luminosity or far too dim for its size.
“This is a ghost of a galaxy. Objects as diffuse as Antlia 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data,” said lead author Dr. Gabriel Torrealba, an astronomer at Taiwan’s Institute of Astronomy and Astrophysics.
Dr. Torrealba and co-authors searched the data from the Gaia mission’s second data release for Milky Way satellites by using RR Lyrae stars — old and metal-poor, typical of those found in dwarf galaxies.
“RR Lyrae had been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disk, we weren’t totally surprised,” said co-author Dr. Vasily Belokurov, researcher at Cambridge’s Institute of Astronomy.
“But when we looked closer at their location on the sky it turned out we found something new, as no previously identified object came up in any of the databases we searched through.”
Using the 3.9-m Anglo-Australian Telescope, the astronomers measured the spectra of more than 100 red giant stars in the ghostly object they spotted and confirmed that it was real — all the stars were moving together.
The team was also able to obtain the galaxy’s mass, which was much lower than expected for an object of its size.
“The simplest explanation of why Antlia 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 Dr. Sergey Koposov, a scientist at 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.”
A paper reporting this discovery will be published in the Monthly Notices of the Royal Astronomical Society.
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G. Torrealba et al. 2018. The hidden giant: discovery of an enormous Galactic dwarf satellite in Gaia DR2. MNRAS, in press; arXiv: 1811.04082
