Most spiral galaxies have a flat, rotating disk that hosts stars, gas and dust, as well as some specific types of edges. Our outstandingly beautiful Milky Way is a different kind of barred spiral galaxy; rather than a plain disc, it has a knot in its backbone, a peculiarity among this type of cosmic bodies.
The cosmic body is dramatically twisted around the edges, an oddity that has been stated by two different studies and has been puzzling astronomers for a long time now. Now, a new paper assaying data from the Gaia expedition has pointed towards a possible explanation: it is the outcome of a clash with a smaller galaxy earlier in the Milky Way’s life.
Researchers, though, do not know when the collision took place, neither which galaxy clashed with it. However, the way the twists move around the galactic nuclei suggests that the event could have been caused by a rather recent, or even ongoing, ruckus with one of the Milky Way’s satellite galaxies.
Gaia Mission Made the Discovery Possible
The Gaia mission has gathered incredible data that revealed the galaxy‘s rather dramatic past. A crash with another galaxy about eight to 11 billion years ago bloated the Milky Way’s dense disk, stuffing it with stars. A collision with a ghost galaxy a few million years back left wrinkles in the Milky Way’s hydrogen, and the conflict with the galaxy known as Gaia Sausage left stars moving in odd orbits.
The Gaia satellite was sent to space in 2013 and had been gathering information ever since in order to create the most detailed 3D map of the Milky Way. It is thoroughly observing the proper movements, radial speeds and distances of the stars to figure out where everything is, and how it is traveling.
“It’s like having a car and trying to measure the velocity and direction of travel of this car over a very short period of time and then, based on those values, trying to model the past and future trajectory of the car,” said astronomer Ronald Drimmel from the Turin Astrophysical Observatory in Italy.
“If we make such measurements for many cars, we could model the flow of traffic. Similarly, by measuring the apparent motions of millions of stars across the sky, we can model large scale processes such as the motion of the warp.”
Another Closely-Located Galaxy Might Be the Culprit
As a team of researchers was examining this data for 12 million stars, they discovered that the twist in the Milky Way’s disk is not located in one place alone. In fact, it is moving around the galactic nuclei, similar to what the stars do, but at a different speed.
Although that speed is slower than that of the stars, it is much more accelerated than other prior explanations for the twist.
“We measured the speed of the warp by comparing the data with our models. Based on the obtained velocity, the warp would complete one rotation around the center of the Milky Way in 600 to 700 million years,” said astronomer Eloisa Poggio from the Turin Astrophysical Observatory. “That’s much faster than what we expected based on predictions from other models, such as those looking at the effects of the non-spherical halo.”
This means that something more gigantic must have caused the warp, such as another galaxy colliding with the Milky Way. The team of astronomers believes that the culprit could be the Sagittarius Dwarf Spheroidal Galaxy. The object has a close orbit with the Milky Way, and it is known to be rather riotous.
Further observations, as well as the next launch of Gaia data set for this year, could determine if this particular galaxy is the one messing with the Milky Way. The study has been published in the journal Nature Astronomy.