Seeing within the Darkish | by Brian Koberlein



27 November 2021

Artist concept showing a hypervelocity star escaping our galaxy.
NASA, ESA, and G. Bacon (STScI)
Artist idea displaying a hypervelocity star escaping our galaxy.

Darkish matter is notoriously troublesome to review. It’s primarily invisible to astronomers since it will probably’t be seen straight. So astronomers depend on results such because the gravitational lensing of sunshine to map its presence within the universe. That technique works effectively for different galaxies, however not so effectively for our personal. To map darkish matter within the Milky Approach, we rely totally on the motions of stars in our galaxy. Since darkish matter attracts common matter gravitationally, the tactic works effectively for areas of the galaxy the place there are stars. Sadly, many of the stars lie alongside the galactic airplane, making it troublesome to map darkish matter above and under that airplane. However a latest examine proposes a approach to map extra of our galaxy’s darkish matter utilizing runaway stars.

Most stars within the Milky Approach are gravitationally certain. This implies they are going to spend their complete life within the Milky Approach. They’ll pace across the galaxy, orbiting the galactic middle like our Solar, however they don’t transfer quick sufficient to ever escape from the gravitational pull of the galaxy. However some stars do have sufficient pace to flee. They’re often called runaway stars, or hypervelocity stars. Both by an in depth encounter with a black gap, or maybe a supernova, they’ve gained super pace, and are on their approach to depart the Milky Approach. Thankfully for this newest examine, hypervelocity stars typically have a path that takes them away from the galactic airplane. So we are able to examine how darkish matter impacts them to map darkish matter in our galaxy.

We think dark matter surrounds the Milky Way in a halo.
L Jaramillo and O Macias, Virginia Tech
We predict darkish matter surrounds the Milky Approach in a halo.

There’s a catch, nevertheless. The pace of every hypervelocity star relies upon totally on the interplay that gave them a kick. We will’t merely have a look at a star and say the sooner it’s transferring the much less darkish matter is close by. As a substitute, the staff appeared on the distribution of hypervelocity speeds and instructions to provide a statistical view of darkish matter. So if statistically hypervelocity stars have a tendency to maneuver extra slowly in a selected course, that may inform us concerning the distribution of darkish matter.

Sadly, there are solely a pair dozen identified hypervelocity stars, which isn’t sufficient to make a very good darkish matter map. So the staff created a simulated pattern of hypervelocity stars primarily based on whether or not darkish matter surrounds the Milky Approach in a sphere, a flattened ellipsoid, and different shapes. They discovered that the samples we presently have are in step with a symmetric distribution of darkish matter (sphere or ellipsoid) and that the form might be additional pinned down with a pattern of solely 400 – 800 hypervelocity stars. That’s way over we presently have, however new telescopes and sky surveys ought to detect these numbers sooner or later.

Hypervelocity stars aren’t the one software we have now to map darkish matter within the galaxy, however as this examine reveals they could be a highly effective software. It offers us additional motivation to search out and monitor these rushing stars. They may simply have another lesson to show us earlier than they depart the Milky Approach eternally.

Supply hyperlink