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Posted on May 18, 2019
A huge "ghostly" galaxy, considered one of the oldest in the universe, was detected on the outskirts of the Milky Way in November 2018 by a team of astronomers who discovered the 39; massive object by digging into new data from Europe Satellite Gaia of the Space Agency. The object, named Antlia 2, escaped detection due to its extremely low density and perfect hiding in the avoidance zone, behind the shroud of the Milky Way disk, a region full of dust and a glut bright stars near the galactic center.
"It's a galaxy ghost," said Gabriel Torrealba, astrophysicist at the Sinica Academy of the Taiwan Academy of Astronomy and Astrophysics (ASIAA) and lead author of the paper. "Objects as diffuse as Ant 2 have not been seen before. Our discovery was only possible thanks to the quality of the Gaia data. "Gaia is able to dig into the avoidance area," he says, "as it provides the correct high-quality motions of the stars behind the central disk of our galaxy, the Milky Way. In other words, he is able to follow the stars moving through the celestial sphere.
Optically, the avoidance area is like "trying to look through a velvet cloth – black as black can be," says Thomas Dame, director of the Harvard Center for Astrophysics radio telescope center. -Smithsonian and astronomer senior Smithsonian radio. Astrophysical Observatory. "In terms of tracing and understanding the spiral structure, half of the Milky Way is terra incognito."
"This is the most important thing in astrophysics" – the "Holy Grail" of astronomy is to give a clear perspective of our relationship to the physical universe. The map of our galaxy of the Milky Way is part of it, a map still incomplete. Our solar system drifts between two spiral arms at its outer edges, some 27,000 light-years away from its center. Beyond that, like ancient sailors navigators, no spaceship has ever traveled beyond the opaque central disk to turn around and take his picture.
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"The avoidance area is basically the part of the sky darkened by the Milky Way disk seen from Earth," Torrealba said. "The Milky Way disk contains a lot of gasoline and stars, which makes it extremely cluttered and complex." But the team was able to use a hundred pulsating stars, called "RR Lyrae" and poor in metal, to probe in and eventually identify Antlia 2.
Optically, entering the avoidance area, it's like "trying to look through a velvet garment – black as black as can be," says Thomas Dame, director of the radio telescope data center. Harvard-Smithsonian Astrophysical Center and Senior Astronomer Radio Smithsonian Astrophysical Observatory. "In terms of tracing and understanding the spiral structure, half of the Milky Way is terra incognito."
"This is the most important thing in astrophysics" – the "Holy Grail" of astronomy is to give a clear perspective of our relationship to the physical universe. The map of our galaxy of the Milky Way is part of it, a map still incomplete. Our solar system drifts between two spiral arms at its outer edges, some 27,000 light-years away from its center. Beyond that, like ancient sailors navigators, no spaceship has ever traveled beyond the opaque central disk to turn around and take his picture.
A swarm of weak dwarf galaxies orbiting the Milky Way – "many more hidden, still to be discovered"
"Compared to the rest of the sixty or so satellites of the Milky Way, Ant 2 is a mirror ball," said co-author 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 large population of almost invisible dwarfs, similar to this one."
Torrealba says that Antlia 2 is probably one of the oldest dwarf galaxies in the universe, but he and his colleagues are still questioning how it has become so diffuse. "One of the possibilities is that Antlia 2 was much more massive in the past and, falling into the Milky Way, it lost its mass to become more diffuse," said Torrealba. A problem with this idea Torrealba says is that rather than grow, galaxies tend to shrink at the same time as they lose stars.
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According to astronomer Sergey Koposov of Carnegie Mellon University, the object of the giant size of the object presents a puzzle in agreement with Torrealba. "Normally, as galaxies lose mass because of the tides of the Milky Way, they shrink and do not grow."
"Another possible explanation for the extraordinary appearance of Antlia 2," wrote Koposov in an email to dailygalaxy.com, "is that there is a problem with the currently favored theory of dark and cold matter that predicts that dark matter should be packed in the centers of galaxies. . If the distribution of dark matter is more mellow, it can facilitate the formation of galaxies like Antlia 2, "he added.
Ant 2 is known as 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 mass 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 it gives off. Compared to LMC, another satellite of the Milky Way, Ant 2 is 10,000 times weaker. In other words, it is far too big for its brightness or too dark for its size.
ESA's Gaia mission has produced the richest catalog of stars to date, including high-precision measurements of nearly 1.7 billion stars and revealing unparalleled details of our galaxy. Earlier in 2018, Gaia's second data release gave scientists around the world new insights into the stars of the Milky Way.
The researchers behind this study – from Taiwan, the United Kingdom, the United States, Australia, and Germany – searched for new Gaia data from the Milky Way satellites with the help of RR Lyrae stars. These stars are old and poor in metals, typical of those found in a dwarf galaxy. RR Lyrae changes their brightness with a half-day period and can be localized through these well defined impulses.
"RR Lyrae had been found in all known dwarf satellites, so we were surprised by a group of them sitting on top of the Galactic disk," said Vasily Belokurov, co-author of the Institute. of Cambridge Astronomy. "But when we took a closer look at their location in the sky, we found something new because no previously identified objects were in any of the databases we consulted."
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 an opportunity window. limited to obtain tracking data. They were able to measure the spectrum of more than 100 giant red stars just before Earth's movement around the Sun made Ant 2 unobservable for months.
The specters allowed the team to confirm that the ghostly object he had spotted was real: all 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 mass of the galaxy, much lower than expected for an object of this size.
If it is impossible to inflate the dwarf by removing matter, then Ant 2 must be born huge. The team has not yet discovered the exact process that has made Ant 2 so extensive. Although current models of galaxy formation have not predicted objects of this size and brightness, it has recently been hypothesized that some dwarfs might be inflated by a formation of light. vigorous stars. Stellar winds and supernova explosions would repel unused gas, weakening the gravity that binds the galaxy and allowing the dark matter to drift too.
"Even though star training could reshape the distribution of dark matter in Ant 2 as it was conceived, it must have acted with unprecedented efficiency," said co-author, Jason Sanders, also from Cambridge.
Alternatively, the low density of Ant 2 could mean that a change in the dark matter properties is needed. The presently privileged theory predicts that dark matter will dwell closely in the center of galaxies. Given the lightness of the new dwarf, it may be necessary to use a dark matter particle less likely to cluster.
The gap between Ant 2 and the rest of the galactic dwarfs is so large that it may well indicate that significant physics is absent from the formation patterns of dwarf galaxies. Solving the Ant 2 puzzle could help researchers understand how early structures of the primitive universe came into existence.
The Daily Galaxy via Imperial College London
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