Milky Way still reeling from impact with "ghost galaxy"
Last year astronomers discovered a gigantic "ghost" galaxy, named Antlia 2, orbiting the Milky Way. Now, new research led by Rochester University has found that the bizarre galaxy may have been involved in a hit-and-run that left the Milky Way with a wobbly galactic disc.
From our perch inside the Milky Way, it's hard to get a sense of the large-scale structure of the galaxy. We know that it's mostly a flat disc with a bulging center, and if you look at it from "above" you'd see it's made up of spiral arms of stars and gas.
But recently it was discovered that the galactic disc also seems to be rippled vertically, creating a texture like corrugated iron. It was suggested that this effect could have been caused by the gravitational aftershocks of a past collision, as a dwarf galaxy swept through the Milky Way's disc. And now, Rochester researchers believe they've identified which dwarf galaxy is responsible.
Only discovered in 2018 in data from ESA's Gaia mission, Antlia 2 is an unusual dwarf galaxy. It's orbiting the Milky Way at a distance of at least 130,000 light-years, and while it's relatively large for a dwarf galaxy it's also incredibly faint for its size. This gigantic "ghost" galaxy was tricky to spot, not only because of its dimness, but because it happened to be hiding behind the bulge of stars at the Milky Way's center.
Now, the Rochester team has pointed the finger at Antlia 2 for the drive-by that shook up the Milky Way. The researchers used Gaia spacecraft data to calculate the past trajectory of the dwarf galaxy, and found that its path likely took it through our galaxy's disc a few hundred million years ago – very recently, in cosmological terms. Further evidence can be found in the fact that Antlia 2's current position closely matches an earlier simulation of where the culprit for the disturbance would be by now.
The team also modeled what kind of effects such a collision would have on the Milky Way. Sure enough, the simulations showed very similar perturbations to those observed in the Milky way disc.
The researchers backed up the finding by considering other suspects, like the other dwarf galaxies orbiting the Milky Way. It was found that the tidal strength of Sagittarius was too weak, and the Large and Small Magellanic Clouds are too far away to create the effects observed.
Of course, the impact would have left a mark on Antlia 2 itself as well. The researchers have also calculated what kinds of motions the stars in the ghostly dwarf galaxy would have after side-swiping the larger galaxy. These predictions can be tested in future Gaia data runs and, if they prove accurate, add even more weight to the idea.
The research was published in the Astrophysical Journal Letters.
Source: Rochester University