A massive invisible mass may be the reason why the Milky Way's disc is warped and twisted.

A new study shows that the only explanation for all the features of the Milky Way's shape is a warped, misaligned dark halo - the huge block of dark matter that surrounds and penetrates our galaxy.

It's hard to see the shape of the galaxy you live in. It's as if we could send a telescope into intergalactic space and take a picture of it. So while astronomers have known for some time that the disc was curved, we didn't have enough data to understand what was going on.

It wasn't until Gaia, a space telescope that maps the positions and velocities of stars in the Milky Way with high precision, that astronomers were able to map the Milky Way's distortion in detail.

Astronomers have found strong, accumulating evidence of distortions and flares on the outskirts of the galaxy. But what causes this is unclear. The leading explanation is a past or ongoing interaction with another galaxy.

But according to Han and his CfA colleagues astronomer Charlie Conroy and astrophysicist Lars Henquist, none of the explanations proposed so far have been able to explain both the distortion and the flare.

Last year, however, a team led by Han and Conroy made an interesting discovery. The stellar halo around the galaxy - the diffuse sphere of gas and stars in which the galactic disc rotates - is also skewed.

This led researchers to believe that the Milky Way's dark halo is also highly curved. This is the sphere of dark matter, an otherwise undetectable mass that surrounds most galaxies and interacts gravitationally with normal matter in the Universe.

So they did some simulations and modelling to see if they could reproduce the observed shape of the Milky Way, the distortion, the flare and all that.

They created a galaxy model in which the dark halo is tilted 25 degrees relative to the Milky Way disc and calculated the orbits of stars and gas over a period of 5 billion years. They found that when the dark matter block is tilted, the outer outskirts of the galaxy do indeed curve and glow as we see in Gaia observations of the Milky Way.

Now, this doesn't mean that a galactic interaction is out of the question. In fact, the team says it's likely. But their results do not show that the interaction is ongoing, but that it is in the past. Their simulations showed that a collision with another galaxy can tilt the dark halo significantly, and that the bending of the galactic disc occurs rapidly in a trajectory that follows the bending of the dark halo.

After the event, the tilt of the dark halo slowly returns to normal; in the simulation, after a galactic collision 7 billion years ago, it took about 5 billion years for the dark halo of the Milky Way analogue to drop from 50 degrees to 20 degrees.

"This result suggests that the Milky Way's dark halo was probably more oblique in the past and has now fallen to its current value (~25 degrees)," the researchers write.

 

Source: https://www.sciencealert.com/

It's nothing to worry about, but it could give us some new information about the evolution of the Milky Way over time, clues to which we are slowly piecing together.

"Here we show that a dark halo bent in the same direction as the stellar halo can cause a bend and flare in the Galactic disc with the same amplitude and direction as the data," writes a team led by astrophysicist Jiwon Jesse Han of the Harvard & Smithsonian Center for Astrophysics (CfA).

"These results, together with data from the stellar halo, provide convincing evidence that our Galaxy is embedded in a tilted dark matter halo. This misalignment of the dark halo and the disc holds clues to the formation history of the Galaxy and represents the next step in dynamical modelling of the Galactic potential."