A huge invisible mass could be the reason why the Milky Way’s disk is warped and twisted.
A new study shows that tilted, misaligned dark halo – a large blob of dark matter that surrounds and permeates our home galaxy – is the only explanation so far that explains all the features of the Milky Way’s shape.
It’s nothing to worry about; but it could give us some new information about the evolution of the Milky Way over time, which we are slowly piecing together, trace by trace.
“Here we show that a dark halo tilted in the same direction as the stellar halo can induce a warp and a flare in the galactic disc with the same amplitude and orientation as the data.” written by the team led by astrophysicist Jiwon Jesse Han of the Harvard & Smithsonian Center for Astrophysics (CfA).
“These results, combined with data in the stellar halo, provide compelling evidence that our Galaxy is embedded in a tilted dark matter halo. This misalignment of the dark halo and disk is a clue to the formation history of the Galaxy and represents the next step in dynamical modeling of the galactic potential.”
It’s hard to see the shape of the galaxy you happen to live in. It’s not like we can send a telescope into intergalactic space to take a photo. So while astronomers have known for some time that the disk was twisted, we didn’t have enough data to figure out what was going on.
It wasn’t until Gaiaa space telescope mapping the positions and velocities of stars in the Milky Way with such high precision that astronomers were able to map the strangeness of the Milky Way in detail.
Astronomers found on the edge of the galaxy strong, accumulation of evidence warp and flare. But it is not clear what could have caused it. The main explanation is an interaction, either past or ongoing, with another galaxy.
However, according to Han and his colleagues, astronomer Charlie Conroy and astrophysicist Lars Henquist, both also of CfA, none of the explanations proposed so far explained both the warp and the flare.
Last year, however, a team led by Han and Conroy made an interesting discovery. The halo of stars around the galaxy – the diffuse ball of gas and stars in which the galactic disk rotates – is also out.
This suggested to the researchers that the Milky Way’s dark halo is also highly tilted. This is the ball of dark matter that wraps around most galaxies, the otherwise undetectable matter that gravitationally interacts with the normal matter in the universe.
So they did some simulations and modeling to see if they could reproduce the observed shape of the Milky Way, warp, flare and so on.
They created a model of a galaxy in which the dark halo was tilted 25 degrees relative to the disk of the Milky Way, and calculated the orbits of the stars and gas over a period of 5 billion years. They found that when the dark matter blob is tilted, the edges of the galaxy actually deform and flare up, just as we see in Gaia observations of the Milky Way.
However, this does not mean that galactic interactions are not involved. In fact, it’s likely, the team says. But their results suggest that the interaction was past, not ongoing. Their simulation showed that a collision with another galaxy can significantly tilt the dark halo, and that the deformation of the galactic disk occurs quickly – within one orbit of the dark halo being tilted.
After the event, the tilt of the dark halo slowly returns to normal; in the simulation after the galactic collision 7 billion years ago, it took about 5 billion years for the dark halo of the Milky Way analog to decrease from 50 degrees to 20 degrees.
“This result,” the researchers write“shows that the Milky Way’s dark halo was probably more inclined in the past and has now decreased to its current value (~25 degrees).
The research was published in Astronomy of nature.
