Scientists studying data from the space ship of the European Space Agency "Gaia" have uncovered a previously unknown patulic galaxy hidden outside the Milky Way, an extremely low density star of two-thirds of the size of the Earth's galaxy. The so-called "ghost" galaxy, known as Antlia 2, is one third of the size of the Milky Way, large like the Great Magellanic Cloud.
How long has it remained unknown? Ant 2 is 10,000 times lighter than the Great Magellanic Cloud and is hidden behind the disk of the Milky Way. It can only be found using Gaia's high precision data and presents a mystery to astronomers: it is too big for its brightness or is too dark in size.
"This is the galaxy's ghost," said Gabriel Torrealba, the lead author of on-line paper describing the discovery. "Objects that diffuse as Ant 2 were simply not seen before. Our discovery was only possible thanks to the quality of the Gaia data."
Gaia collected high precision measures on millions of stars across the Milky Way. The researchers used this data to look for old RR Lirae stars, which had poor metallicity, typical Denizen dwarf galaxies, which pulsate or change brightness every 12 hours.
"RR Lirae was found on every known satellite, so when we found a group of them that were above the Galactic disk, we were not completely surprised," said co-author Vasiliy Belokurov of the Cambridge Institute of Astronomy. "But when we looked closer to their location in the sky, it turned out that we found something new, because in any database we searched did not reach the previously identified object."
The team confirmed the discovery of Ant 2 after collecting spectra that show the stars move together. But unlike the typical spirit, there is nothing terrible about Ant 2: data show that the dwarf galaxy always stays about 130,000 light-years away from the Milky Way.
"The simplest explanation of why Ant 2 is so small today is that it is separated by the galactic tides of the Milky Way," co-author Sergei Koposov of the Carnegie Mellon University said. "However, what remains inexplicable is the size of the object giant. Usually, since the galaxies are losing mass in the tide of the Milky Way, they are decreasing, not growing."
Co-authored Matthev Valker, also from Carnegie Mellon: "Compared to the other 60 or more satellites on the Milky Way, Ant 2 is weird. We wonder if this galaxy is just the tip of the iceberg, and the Milky Way is surrounded by a large a population of almost invisible dwarfs similar to this. "