It's both massive and extremely diffuse.
Around and Around
Seems like everything in the universe is orbiting something. The Moon orbits the Earth, which orbits the Sun, which orbits the center of the Milky Way.
But it doesn't stop there. Galaxies can even orbit other galaxies, and now, an international team of astronomers has discovered a new satellite galaxy around our own Milky Way — and it's a weird one.
According to a paper published in arXiv earlier in November, the researchers discovered this new dwarf galaxy, which they call Antlia 2 — or "Ant 2" — while sifting through data collected by the European Space Agency’s Gaia satellite.
As for how an entire galaxy escaped detection for so long, especially given its close proximity to our own, a couple of factors come into play. For one, Ant 2 is located behind the Milky Way’s disc — an ideal hiding spot. It's also extremely low in density, giving off very little light.
"This is a ghost of a galaxy," researcher Gabriel Torrealba said in a press release. "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."
Ant 2 isn't the only galaxy known to orbit our own, but it is "an oddball," according to researcher Matthew Walker. It's relatively massive — roughly a third of the size of the the Milky Way — and the researchers are perplexed as to how it might have gotten that way.
"Normally, as galaxies lose mass to the Milky Way’s tides, they shrink, not grow," researcher Sergey Koposov said in the press release.
Knowing that this unusual galaxy exists has left the astronomers curious about what else they might be missing in our galactic backyard. “We are wondering whether this galaxy is just the tip of an iceberg," said Walker, "and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one.”
READ MORE: Gaia Spots a ‘Ghost’ Galaxy Next Door [University of Cambridge]