The Gaia satellite has spotted an enormous “ghost” galaxy lurking on the outskirts of the Milky Way. An international team of astronomers discovered the massive object when trawling through data from the European Space Agency’s (ESA) Gaia satellite. The object, named Antlia 2 (or Ant 2), has avoided detection until now thanks to its extremely low density as well as a perfectly-chosen hiding place, behind the shroud of the Milky Way’s disk. The researchers published the study results online November 9, 2018.
Ant 2 is known as a dwarf galaxy. 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 most other known dwarf satellites of our Milky Way galaxy, Ant 2 is immense: it is as big as the Large Magellanic Cloud, and a third the size of the Milky Way itself.
The researchers say that Ant 2 never comes too close to the Milky Way, always staying at least 40 kiloparsecs (about 130,000 light-years) away. What makes Ant 2 even more unusual is how little light it gives out. Compared to the Large Magellanic Cloud, which is 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.
This is a ghost of a galaxy. 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.
The Gaia data allowed the researchers to obtain the galaxy’s mass, which was much lower than expected for an object of its size.
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. 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.
Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball. 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.
The ESA’s Gaia mission has produced the richest star catalogue to date, including high-precision measurements of nearly 1.7 billion stars, and revealed previously unseen details of our Milky Way galaxy. Earlier this year, Gaia’s second data release made new details of stars in the Milky Way available to scientists worldwide.
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