Ancient Ghost Galaxy Hidden In the Milky Way’s ‘Zone of Avoidance’

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. The object, named 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.   

Revealed by Gaia Spacecraft Data

“This is a ghost of a galaxy,” said Gabriel Torrealba, an astrophysicist at ObjecTaiwan’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. That is, it is able to track stars as they move across the celestial sphere.

In the image above, Antlia 2 is the faint galaxy on the right; Milky Way is center; and the LMC is shown 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. For example, although the Kepler Supernova in 1604 was the most recent supernova to be directly observed in our Galaxy, astronomers recently detected via X-rays and radio waves a supernova remnant that is only 140 years old near the galactic center, which was missed in optical wavelengths due to the large concentration of dust toward its line of sight (Nature).

The map of our Milky Way is a map that is still 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. The map of our Milky Way galaxy is a part of that, a map that is still incomplete. Our solar system drifts 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 spacecraft has ever traveled beyond the opaque central disk to turn back and take its picture.

Like trying to peer through a velvet cloth

“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 in the universe

Torrealba says that Antlia 2 is likeliest one of the oldest dwarf galaxies in the universe, but he and colleagues are still puzzled as to 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. One problem with this idea, says Torrealba, is that rather than grow, galaxies tend to shrink at the same time they lose stars.

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.”

Dark matter distribution

“Another possible explanation of the extraordinary appearance of Antlia 2,” Koposov wrote in an email to, “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.

Dwarfs were the first galaxies in the Universe

As structures emerged in the early Universe, dwarfs were the first galaxies to form, and so most of their stars are old, low-mass and metal-poor. But compared to the other known dwarf satellites of our Galaxy, Ant 2 is immense: it is as big as the Large Magellanic Cloud (LMC), and a third the size of the Milky Way itself.

Ant 2 –far too large for its luminosity or far too dim for its size

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. In other words, it is either far too large for its luminosity or far too dim 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 old and metal-poor, typical of those found in a dwarf galaxy. RR Lyrae change their brightness with a period of half a day and can be located thanks to these well-defined pulses.

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 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.

Ant 2 was born huge

If it is impossible to puff the dwarf galaxy up by removing matter from it, then Ant 2 must have 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, recently it has been speculated that some dwarfs could be inflated by vigorous star formation. Stellar winds and supernova explosions would push away the unused gas, weakening the gravity that binds 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.

Important physics may be missing

Alternatively, 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 keen to 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 is missing in the models of dwarf galaxy formation. Solving the Ant 2 puzzle may help researchers understand how the first structures in the early universe emerged.

**By Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Imperial College London