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Posted on November 13, 2018
What remains unexplained is the giant size of the object, says astronomer Sergey Koposov of Carnegie Mellon University. "Normally, as galaxies lose mbad against the tides of the Milky Way, they shrink and do not grow." the Milky Way
An international team of astronomers, including from the University of Cambridge, discovered this large object when searching for data through the Gaia satellite of the European Space Agency . The object, named Antlia 2 (or Ant 2), has not been detected until now thanks to its extremely low density and its perfectly chosen hiding place, behind the veil of the Milky Way disk. The researchers published their findings online today.
Ant 2 is a dwarf galaxy. When structures emerged at the beginning of the Universe, the dwarves were the first galaxies to form. Most of their stars are therefore aged, of low mbad and poor in metals. But compared to other known dwarf satellites in our galaxy, Ant 2 is huge: it is as big as the Large Magellanic Cloud (LMC) and one-third the size of the Milky Way itself.
What makes Ant 2 even more unusual is the little light that it diffuses. Compared to LMC, another satellite of the Milky Way, Ant 2 is 10,000 times lower. In other words, it is either too big for its brightness, or too dark for its size.
"It's a galaxy ghost," said Gabriel Torrealba, lead author of the document. "Objects as diffuse as Ant 2 have not been seen before. Our discovery was only possible thanks to the quality of the Gaia data. "
ESA's Gaia Mission has produced the richest star catalog to date, including high-precision measurements of nearly 1.7 billion stars and revealing unpublished details of our country of origin. . Earlier this year, Gaia's second data release gave scientists around the world new information about the stars of the Milky Way.
for the Milky Way satellites using the RR Lyrae stars. These stars are old and poor in metals, typical of those found in a dwarf galaxy. RR Lyrae changes brightness in half a day and can be located with these well-defined pulses.
"RR Lyrae was found in all known dwarf satellites. So we found a group of them sitting above the Galactic Disk, we were not totally surprised, "said co-author Vasily Belokurov of the Institute for Scientific Research. Cambridge astronomy. "But when we took a closer look at their position in the sky, we found something new, no previously identified objects having appeared in any of the databases we consulted."
L & # The team contacted colleagues from the Anglo-Australian telescope. (AAT) in Australia, but after checking the contact details of Ant 2, they realized that they had little time to get tracking data. They were able to measure the spectrum of more than 100 giant red stars just before the movement of the Earth around the Sun made Ant 2 unobservable for months.
Spectrum allows the team to confirm that the ghostly object he spotted is real: the stars were moving together. Ant 2 never gets too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years). The researchers were also able to get the mbad of the galaxy, much lower than expected for an object of this size.
"The simplest explanation of why Ant 2 seems to have so little mbad today is that it was dismantled by the galactic tides of the Milky Way," said co-author Koposov . "What remains unexplained, however, is the giant size of the object.Normally, as the galaxies lose mbad because of the tides of the Milky Way, they shrink and do not grow." [19659003] If it is impossible to inflate the dwarf by removing matter, then Ant 2 must have been born huge.The team has not yet discovered the exact process that made Ant 2 so extensive. Current models of galaxy formation have not predicted objects of this size and brightness, it has been recently hypothesized that some dwarfs might be inflated by vigorous star formation. Stellar winds and supernova explosions would repel unused gas, thus weakening the gravity that binds the galaxy and allowing the dark matter to drift too.
"Even though star formation could reshape the distribution dark matter in Ant 2 it was set up, it must have acted with unprecedented efficiency, "said co-author Jason Sanders, also of Cambridge.
The low density of Ant 2 may also require a modification of the dark matter properties. The presently privileged theory predicts that dark matter will dwell closely in the center of galaxies. Given the mellifluous appearance of the new dwarf, a dark matter particle less likely to cluster may be needed.
"Compared to the rest of the 60 satellites around the Milky Way, Ant 2 is a funny balloon," said co-writer Matthew Walker, also of Carnegie Mellon University. "We wonder if this galaxy is only the tip of an iceberg and if the Milky Way is surrounded by a vast population of almost invisible dwarfs, similar to this one. is so wide that it may well indicate that important physical models are missing in dwarf galaxy formation models. Solving the Ant 2 puzzle could help researchers understand how early structures of the early universe came into existence. Finding more objects like Ant 2 shows just how common such ghostly galaxies are and the team is looking for other similar galaxies in Gaia data.
The Daily Galaxy via the University of Cambridge
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