Map of Milky Way halo reveals dark matter ocean
Astronomers observed distant stars in the faint halo surrounding our Milky Way galaxy and have now created a map of the halo, the first of its kind of these outermost parts of our galaxy. These new observations, the astronomers said in April 2021, show how the Large Magellanic Cloud – one of the Milky Way’s satellite galaxies – has created a wake, like a ship sailing through calm waters, as it travels through the Milky Way’s halo. The wake shows up as a distinct bright pathway of stars on the map, telling us that the Magellanic Clouds are still traveling in their very first orbit around the Milky Way galaxy. And the wake itself may be made up of dark matter, dragging the stars along with it!
There are many interesting things about galactic haloes. They are faint and hard to observe, extend to large distances out from their galaxies and are – apart from a few stars, gas and dust – thought to contain a large amount of dark matter. Dark matter is called “dark” not only because we know little about it, but because it does not reveal its existence to us through light, only through gravitational interaction with other matter. The video above illustrates how far out from our galaxy’s main disk the halo stretches, as well as the pathway – the wake – of the Large Magellanic Cloud traveling through it.
If dark matter indeed does make up most of the halo – and all different theories on the nature of dark matter agree on that – a galaxy traveling through the halo would also leave a wake in the dark matter, not only the stars. As a NASA/JPL statement described it:
The wake observed in the new star map is thought to be the outline of this dark matter wake; the stars are like leaves on the surface of this invisible ocean, their position shifting with the dark matter.
The inner regions of the Milky Way halo have already been investigated in detail, but this is the first time astronomers have been able to similarly map the outer regions of the halo, including the wake, at a distance of 200,000 to 325,000 light-years from the center of our galaxy. (As a comparison, the visible Milky Way galaxy disk that we are more familiar with has a diameter of about 100,000 light years, so this is very far out indeed).
With the halo being so faint, how do you go about observing it? Although the stars are extremely sparse in the halo, there are still some there. The researchers measured 1,301 stars located at the vast distances of the halo, using data from the European Space Agency’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer (NEOWISE, which also gave name to the comet of 2020). But accurately pinpointing their distances was one of the major hurdles. So they picked only a specific kind of red giant stars, classified as K giants in the stellar classification scheme. NEOWISE could efficiently detect these stars in the infrared part of the electromagnetic spectrum, which helped the team find their precise distances in the halo and create the map.
Part of the team behind this research had predicted how the dark matter in the Milky Way halo should look like, using computer models. So when the observational data showed a wake behind the Large Magellanic Cloud and another higher density region of stars in the northern part of the halo, this was not entirely a surprise.
Team member Gurtina Besla at University of Arizona’s Steward Observatory said:
What has been a purely theoretical prediction has now been validated by observational data, providing a compelling argument for the existence of dark matter.
Lead author Charlie Conroy, professor at Harvard University, described how we can learn more about dark matter, such as what it consists of, through combining models and data:
You can imagine that the wake behind a boat will be different if the boat is sailing through water or through honey. In this case, the properties of the wake are determined by which dark matter theory we apply.
Nicolás Garavito-Camargo, a co-author of the study at University of Arizona, explained how this research applies to other galaxies as well:
The Milky Way is the only galaxy in which we can resolve the stars and the halo to this level of detail, so it is our most important ‘natural laboratory’ in which we can study how galaxies work in general. We think that what we observe here likely applies to similar galaxies throughout the universe.
The Large Magellanic Cloud is a small galaxy rotating around the Milky Way, about 160,000 light years away from us. It and its smaller companion, the Small Magellanic Cloud (often abbreviated LMC and SMC, respectively), are clearly visible with the unaided eye from the Southern Hemisphere, where they look exactly like their namesake: like curious stationary clouds. LMC is predicted to collide with the Milky Way in the distant future, and in essence, this collision has already started if you take the halo into account as a part of our galaxy.
Bottom line: Astronomers have created a map of the halo of our Milky Way galaxy – its far outer regions – showing how the Large Magellanic Cloud has created a wake along its traveled path, evidence that the satellite dwarf galaxy is only on its very first orbit around the Milky Way. The map provides a way to learn more about the nature of dark matter, thought to compose a large part of the galactic halo.