Astronomers have discovered an immense “ghost galaxy” very close to the Milky Way.
The low-density, extremely old galaxy, now dubbed Antlia 2, is massive but until now has gone completely undetected, despite being – in cosmological terms – right on our doorstep.
Actually, however, perhaps “right outside the back door” is a more apt description. In a paper published on the pre-print site arXiv, a team of astronomers led by Gabriel Torrealba of Academia Sinica in Taiwan notes that Antlia 2 is almost perfectly hidden behind the shroud of the Milky Way’s disc.
It is also ancient, comprising stars that are old, low-mass and metal-poor, meaning that it is a long way from being the brightest thing in the neighbourhood. Indeed, compared to another Milky Way satellite galaxy, the Large Magellanic Cloud, it is 10,000 times fainter.
Torrealba and colleagues from the UK, US, Australia and Germany only discovered Antlia 2 after poring through data collected by the European Space Agency’s Gaia satellite. The mission has so far provided high-precision measurements of 1.7 billion stars, with the results made available to scientists worldwide.
“This is a ghost of a galaxy,” says Torrealba. “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.”
Antlia 2 is formally classified as a dwarf galaxy, even though it is huge, about one-third the size of the Milky Way. The terminology refers to age rather than size, denoting galaxies that formed early in the history of the universe and are typically quite dim.
The researchers first realised that something immense lurked just outside our own home when they scoured the Gaia data looking for a particular type of pulsing star known as RR Lyrae, considered firm indicators of the presence of dwarf galaxies.
“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,” explains co-author Vasily Belokurov.
“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.”
Seeking more information, Belokurov and colleagues contacted the Anglo-Australian Telescope (AAT) in Australia and asked researchers there to zero in on the coordinates.
There was only a limited opportunity to do so before the movement of the Earth around the sun rendered the area unobservable for months. In the time remaining, the AAT scientists succeeded in measuring more than 100 red giant stars in the region.
The data revealed an enormous, but very dispersed, galaxy about 130,000 light-years away from the Milky Way.
“The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the galactic tides of the Milky Way,” says co-author Sergey Koposov.
“What remains unexplained, however, is the object’s giant size. Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow.”