Gaia data reveals unseen 'ghost' galaxy near Milky Way – Astronomy Now

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 of the size of Earth's galaxy. The so-called "ghost" galaxy, known as Antlia 2, is a third of the size of the Milky Way, as it's a 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 was only possible to get high-precision data from astronomers for astronomers: it was too broad for its luminosity or 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 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, we are not totally surprised, "said co-author Vasily Belokurov of Cambridge's Institute of Astronomy. "But when we looked closer to their location on the sky, we found that we were able to find a new source of information."

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 Antarctica: The truth is why it's just about 130,000 light years from the Milky Way.

"said co-author Sergey Koposov from Carnegie Mellon University. "What remains unexplained, however, is the object's giant size. Normally, have galaxies lose mbad 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. "