Posted on May 18, 2019
An enormous 'ghost' galaxy, believed to be one of the oldest in the universe, was detected on the outskirts of the Milky Way in November or 201
"This is a ghost of a galaxy," said Gabriel Torrealba, an astrophysicist at Taiwan's Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) and the paper's lead author. “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 is able to get into the Zone of Avoidance, it says, because it provides high-quality clean motions of stars behind the central disk of our Milky Way galaxy. That is, it's possible to track stars as they move across the celestial sphere.
Optically, the Zone of Avoidance is like "trying to look through a velvet cloth — black as black can be," says Thomas Dame, Director of The Radio Telescope Data Center at the Harvard-Smithsonian Center for Astrophysics and Senior Radio Astronomers at the Smithsonian Astrophysical Observatory. "In terms of tracing and understanding the spiral structure, terra incognito is actually half of the Milky Way."
"The most important thing in astrophysics" – the "Holy Grail" of astronomy is to provide a clear perspective of our relationship to the physical universe. The map of our Milky Way galaxy is part of that, a map that is still incomplete. Our solar system operates between two spiral arms at its outer edges, some 27,000 light-years from its center. Beyond that, like the ancient seafaring mariners, no space craft has ever traveled beyond the central disk to turn back and take its picture.
"Monsters & Dragons?" – Mapping the Terra Incognito or Milky Way's Unseen Far Side
"The zone of avoidance is basically the part of the cloud obscured by the Milky Way's disk as seen from the Earth," said Torrealba. "The disk of the Milky Way has a lot of gas and stars, making it extremely crowded and complex." But the team was able to use about a hundred old and metal-poor pulsating, so-called "RR Lyrae" stars to probe Inside and ultimately identify Antlia 2.
Optically, penetrating the Zone of Avoidance is like "trying to look through a velvet cloth — black as black can be," says Thomas Dame, Director of the Radio Telescope Data Center at the Harvard The Smithsonian Center for Astrophysics and Senior Radio Astronomers at the Smithsonian Astrophysical Observatory. "In terms of tracing and understanding the spiral structure, terra incognito is actually half of the Milky Way."
"The most important thing in astrophysics" – the "Holy Grail" of astronomy is to provide a clear perspective of our relationship to the physical universe. The map of our Milky Way galaxy is part of that, a map that is still incomplete. Our solar system operates between two spiral arms at its outer edges, some 27,000 light-years from its center. Beyond that, like the ancient sea-faring mariners, no space craft has ever traveled beyond the opaque central disk to turn back and take its picture.
Swarm or Faint Dwarf Galaxies Orbit the Milky Way – "Many More Hidden, Yet to Be Discovered ”
"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 are wondering if 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."
Torrealba says that Antlia 2 is probably one of the oldest dwarf galaxies in the universe, but he and his colleagues are still puzzled as to how it became so diffuse. "One possibility is that Antlia 2 was much more massive, and as it falls into the Milky Way, it lost its mass to become more diffuse," said Torrealba. One problem with this idea says Torrealba is that rather than grow, galaxies tend to shrink at the same time they lose stars.
Extreme Disk Galaxy Discovered – "Seven Times the Width of the Milky Way"
The object's giant size, says astronomer Sergey Koposov at Carnegie Mellon University presents a puzzle, agreeing with Torrealba. "Normally, as galaxies lose mass to the Milky Way's tides, they shrink, not grow."
"Another possible explanation of the extraordinary appearance of Antlia 2," wrote Koposov in an email to dailygalaxy.com, "is that there is something wrong with currently favored Cold Dark Matter theory that predicts that dark matter should be tightly packed in centers of galaxies. If dark matter distribution is more fluffy, it can make it easier to form galaxies like Antlia 2, ”he added.
Ant 2 is known as a dwarf galaxy. As structures emerged in the early Universe, dwarfs were the first galaxies to form, and most of their stars were old, low-mass and metal-poor. But compared to the other known dwarf satellites of our Galaxy, Ant 2 is immense: it's as big as the Large Magellanic Cloud (LMC), and a third the size of the Milky Way itself.
What makes Ant 2 even more unusual is how little light it gives out. Compared to the LMC, another satellite of the Milky Way, Ant 2 is 10,000 times fainter. It is either too large for its luminosity or far too dim for its size.
The ESA's Gaia mission has produced the richest star catalog to date, including high-precision measurements of nearly 1.7 billion stars and revealing previously. Unseen details of our home galaxy. Earlier in 2018, Gaia's second data release made new details of stars in the Milky Way available to scientists worldwide.
The researchers behind the current study – from Taiwan, the UK, the US, Australia and Germany – searched the new Gaia data for Milky Way satellites by using RR Lyrae stars. These stars are old and metal-poor, typically of those found in a dwarf galaxy. RR Lyrae changes their brightness with a period of half a day and can be located thanks to these well-defined pulses. ”RR Lyrae has been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disc, we were totally surprised, ”said co-author Vasily Belokurov from Cambridge's Institute of Astronomy. "But when we looked closer at their location on the sky it turned out we found something new, as previously identified object came up any of the databases we searched through."
The team contacted colleagues at the Anglo-Australian Telescope (AAT) in Australia, but when they checked the coordinates for Ant 2, they realized they had a limited window of opportunity to get follow-up data. They were able to measure the spectra of more than 100 red giant stars just before the Earth's motion around the Sun rendered Ant 2 unobservable for months.
The spectra enabled the team to confirm that the ghostly object they spotted was real: all the stars were moving together. Ant 2 never comes too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years) away. The researchers were also able to obtain the galaxy's mass, which was much lower than expected for an object of its size.
If it is impossible to push the dwarf up by removing matter from it, then Ant 2 had been born huge. The team has yet to figure out the exact process that made Ant 2 so extended. While objects of this size and luminosity have not been predicted by current models of galaxy formation, they have been speculated that some dwarfs could be inflated by vigorous star formation. Stellar winds and supernova explosions would push the unused gas, weakening the gravity that is connected to the galaxy and allowing the dark matter to drift outward as well.
"Even if star formation could re-shape the dark matter distribution in Ant 2 as it was put together, it must have acted with unprecedented efficiency, ”said co-author Jason Sanders, also from Cambridge.
Alternative, Ant 2's low density could mean that a modification to the dark matter properties is needed. The currently favored theory predicts dark matter to pack tightly in the centers of galaxies. Given how fluffy the new dwarf appears to be, a dark matter particle which is less than a cluster may be required.
The gap between Ant 2 and the rest of the Galactic dwarfs is so wide that this may well be an indication that some important physics are missing in the models of dwarf galaxy formation. Solving the Ant 2 can help researchers understand how the first structures in the early universe emerged.
The Daily Galaxy via Imperial College London