An enormous ‘ghost’ galaxy, believed to be one of the oldest in the universe, was detected lurking on the outskirts of the Milky Way in November of 2018 by a team of astronomers who discovered the massive object when trawling through new data from the European Space Agency’s Gaia satellite. Name the object Antlia 2 , avoided detection thanks to its extremely low density as well as a perfect hiding place in the Zone of Avoidance, named by Edwin Hubble in 1929, behind the shroud of the Milky Way’s disc–a region full of dust and an overabundance of bright stars near the galactic center.
Gaia Spacecraft Data Revealed
“This is a ghost of a galaxy,” said Gabriel TorrealbaObjec, a physicistTaiwan’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 dig into the Zone of Avoidance, he says, because it provides high-quality proper motions of stars behind the central disk of our Milky Way galaxy. It is capable of tracking stars as they move through the celestial field.
Above, Antlia 2 can be seen as the faint galaxie on the right. Milky Way is in the center. The LMC can be seen on the left.
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.
Zone of Avoidance–Half of the Milky Way is terra incognito
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 Astronomer at the Smithsonian Astrophysical Observatory. “In terms of tracing and understanding the spiral structure, essentially half of the Milky Way is terra incognito.” Even some of the brightest explosions in our Milky Way are missed if they occur on the other side of our Galaxy. Even though the Kepler supernova in 1604 was the latest supernova directly observed in our Galaxy. Astronomers discovered recently via radio waves and X-rays a supernova remnant only 140 years old close to the galactic center. It was not detected in optical wavelengths as a result of its large concentration of dust.Nature).
The map of the Milky Way is incomplete.
The Incomplete Map
“It’s the most Important thing in astrophysics”–the ‘Holy Grail’ of astronomy is to provide a clear perspective of our relationship to the physical universe. A part of that map, the map of our Milky Way galaxy, is the map. However, it is still incomplete. Our solar system is located 27,000 light-years away from its center, between two spiral arms. A spacecraft that has not yet traveled beyond its opaque central disk has never been able to take its image, just like the seafaring mariners.
“Ghost Galaxies” of a Great Wall Discovered Hidden Beyond the Milky Way
It’s like trying to look through a velvet fabric.
“The zone of avoidance is basically the part of the sky 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.
Ant 2 –”The tip of an iceberg?”
“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 whether 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.”
One of the oldest dwarf galaxies known to exist in the universe
Torrealba believes that Antlia 2 may be the oldest dwarf galaxy in the universe. However, he and his colleagues remain puzzled about how it became so diffuse. “One possibility is that Antlia 2 was much more massive in the past, and as it fell into the Milky Way, it lost its mass to become more diffuse,” said Torrealba. Torrealba points out that instead of growing, galaxies shrink when they lose stars.
The object’s giant size, says astronomer Sergey KoposovCarnegie Mellon University presents a puzzle that agrees with Torrealba. “Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow.”
Dark matter distribution
“Another possible explanation of the extraordinary appearance of Antlia 2,” Koposov wrote 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 however is more fluffy, that can make it easier to form galaxies like Antlia 2,” he added.
The Universe’s first galaxies were discovered by dwarfs
Because dwarfs are the first galaxies that formed in the early Universe, they were also the first to have structures. Ant 2 is a vast satellite, a third of the size of the Milky Way and as large as any of the other dwarf satellites.
Ant 2 –far too large for its luminosity or far too dim for its size
Ant 2’s lack of light makes it even more extraordinary. Ant 2 is 10,000 times fainter than the LMC satellite, which is another satellite of Milky Way. It is either too big for its luminosity, or too small for its size.
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 typical of dwarf galaxy stars. They are old and metallic-poor. These well-defined pulses can help locate RR Lyrae, which change their brightness every half a day.
RR Lyrae Stars
“RR Lyrae had been found in every known dwarf satellite, so when we found a group of them sitting above the Galactic disc, we weren’t 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 no previously identified object came up in any of the databases we searched through.”
The team contacted AAT in Australia and made contact with colleagues. But, when they looked at Ant 2 coordinates, they found that there was only a narrow window of opportunity for them to collect 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 team was able to confirm the ghostly object that they had spotted with spectra: all stars were moving together. Ant 2 is never too close to the Milky Way. It always stays at least 40 kiloparsecs (130,000 light-years) from it. The researchers were also able to obtain the galaxy’s mass, which was much lower than expected for an object of its size.
Ant 2 was a huge success
Ant 2 cannot be inflated by adding matter to the dwarf galaxy. However, the team is still trying to determine how Ant 2 was extended. Although objects this large and bright are not predicted by current models for galaxy formation, it was suggested that some dwarfs might be created by star formation. Supernova explosions and stellar winds would blow away any unused gas. This would weaken the gravity that binds galaxy to the star and allow dark matter to drift further outward.
“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.
It is possible that important physics might be missing
Alternatively, Ant 2’s low density could mean that a modification to the dark matter properties is needed. Current theory predicts that dark matter will be packed tightly in the center of galaxies. Due to the fluffy appearance of the new dwarf, it may be necessary for dark matter particles to cluster less.
The gap between Ant 2 & the other Galactic dwarfs may be a sign that something is wrong with the models of dwarf star formation. Researchers might be able solve the Ant 2 puzzle to help them understand how early structures emerged in the universe.
Maxwell Moe, astrophysicist, NASA Einstein FellowUniversity of Arizona via Imperial College London
Maxwell Moe (astrophysicist, NASA Einstein Fellow University of Arizona), Maxwell Moe Max is often found at Kitt Peak National Observatory two nights per week, exploring the mysteries of the Universe. Max earned his Ph.D. in Astronomy in 2015 from Harvard University.
