The Gaia satellite has detected a huge ghost galaxy that tricks on the outskirts of Winter Street.
An international team of astronomers, including the University of Cambridge, discovered the massive object as they trawled data from the European Space Agency's Gaia satellite. The object, named Antlia 2 (or Ant 2), has avoided detection so far thanks to its extremely low density and a perfectly selected hideaway behind the Milk disc. The researchers have published their results online today.
Ant 2 is known as a dwarf algae. When structures originated at the beginning of the universe, dwarfs were the first galaxies to form, so most of their stars are old, low-mass and low-fat. But compared to the other known dwarf satellites in our galaxy, Ant 2 is huge: it is as big as the Great Magellanic Cloud (LMC) and one third of the size of the size.
What makes Ant 2 even more unusual is how little light it gives. Compared with LMC, another satellite in Winter Street, Ant 2 is 10,000 times fewer. In other words, it's either too big for its luminosity or just too dim for its size.
"This is a galaxy's ghost," says Gabriel Torrealba, the author's main author. "Items that diffuse like Ant 2 simply have not seen before. Our detection was only possible thanks to the quality of Gaia data."
ESA's Gaia mission has so far produced the richest star catalog, including high-precision measurements of almost 1.7 billion stars, revealing previously invisible details in our Galaxy. Earlier this year, Gaia's second computer bill made new details about stars in Winter Street available to researchers around the world.
The researchers behind the current survey – from Taiwan, Great Britain, USA, Australia and Germany – were looking for the new Gaia data for Milky Way satellites using RR Lyrae stars. These stars are old and metallic, typical of those found in a dwarf galaxy. RR Lyrae changes its brightness by half a day and can be placed thanks to these well-defined pulses.
"RR Lyrae was found in all known dwarf satellites, so when we found a group of those sitting above the Galactic disc, we were not completely surprised," said co-author Vasily Belokurov from the Cambridges Institute of Astronomy. "But as we looked closer at their location in the sky, we showed that we found something new, as no previously identified object was found in any of the databases we searched through."
The team contacted colleagues at the Australian Telescope (AAT) in Australia, but when they checked the coordinates of Ant 2, they realized that they had a limited window the ability to get follow-up data. They could measure spectra of more than 100 red giant stars just before Earth's motion around the sun made Ant 2 unimaginative for several months.
Spektra enabled the team to confirm that the spooky object they discovered was real: all the stars were moving together. Myr 2 never gets too close to the Milky Way, always holds at least 40 kiloparsecs (about 130,000 light years) away. The researchers could also get the mass of the galaxy, which was much lower than expected for an object of its size.
"The easiest explanation for why Ant 2 seems to have such a small mass today is that it is removed from Western Galactic tides," said co-author Sergey Koposov from Carnegie Mellon University. "What remains unexplained, however, is the enormous size of the object. Normally, as galaxies lose mass to the tidal water of the winter, they will shrink, do not grow."
If it is impossible to push the dwarf by removing the case from it, then Ant 2 had been born enormously. The team has yet to figure out the exact process that made Ant 2 so expanded. Although objects of this magnitude and brightness have not been predicted by current models of galaxy formation, it has recently been speculated that some dwarfs may be inflated by strong star formation. Stellar winds and supernova explosions would push out the unused gas, weaken the gravity that binds the galaxy and allow the dark material to drift outward as well.
"Although star formation could transform the dark matter distribution into Ant 2, because it was put together, it must have acted with unrivaled efficiency," co-author Jason Sanders, also from Cambridge, said.
Alternatively, Ant 2's low density may cause a modification of the dark material properties. The currently favored theory predicts dark matter packing closely in the center of the galaxy. Considering how fluffy the new dwarf appears to be, a dark particle particle may be less concerned that clusters are necessary.
"Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball," said co-author Matthew Walker, also from Carnegie Mellon University. "We wonder if this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of almost invisible dwarves that are similar to this."
The gap between Ant 2 and the other galactic dwarfs is so wide that it may be an indication that some important physics are missing in the models of dwarf algal formation. Solving the Ant 2 pusset can help researchers understand how the first structures of the early universe emerged. Finding more items like Ant 2 shows how common such ghostly galaxies are, and the team is busy looking for other similar galaxies in Gaia data.
Four newly discovered neighbors
G. Torrealba et al. The hidden giant: The discovery of a huge galactic dwarf satellite in Gaia DR2. arXiv: 1811.04082 [astro-ph.GA]. arxiv.org/abs/1811.04082