Scientists studying data from the European Space Agency’s Gaia spacecraft have discovered a previously unknown dwarf galaxy lurking just outside the Milky Way, an extremely low-density swarm of stars two thirds the size of Earth’s galaxy. The so-called “ghost” galaxy, known as Antlia 2, is one third the size of the Milky Way, as big as the Large Magellanic Cloud.
How did it remain unknown for so long? Ant 2 is 10,000 times fainter than the Large Magellanic Cloud and it is hidden behind the disk of the Milky Way. It could only be found using Gaia’s high-precision data and it poses a mystery for astronomers: it is either too large for its luminosity or far too dim for its size.
“This is a ghost of a galaxy,” said Gabriel Torrealba, lead author of an on-line paper describing the discovery. “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 collected high-precision measurements on millions of stars across the Milky Way. The researchers used that data to look for old, metal-poor RR Lyrae stars, typical denizens of dwarf galaxies, that pulse, or change brightness, every 12 hours or so.
“RR Lyrae had been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disc, we weren’t totally surprised,” said co-author Vasily Belokurov from 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.”
The team confirmed the Ant 2 discovery after collecting spectra showing the stars were moving together. But unlike a typical ghost, there’s nothing scary about Ant 2: the data shows the dwarf galaxy always stays about 130,000 light years from the Milky Way.
“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.”
Said co-author Matthew Walker, also from Carnegie Mellon: “Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball. 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.”