Astronomers using data ESA’s Gaia mission and NASA’s Near Earth Object Wide Field Infrared Survey Explorer (NEOWISE) have created a map of the outermost region of our Milky Way Galaxy.
Images of the Milky Way and the Large Magellanic Cloud are overlaid on a map of the surrounding Galactic halo. The smaller structure is a wake created by the Large Magellanic Cloud’s motion through this region. The larger light-blue feature corresponds to a high density of stars observed in the northern hemisphere of our Galaxy. Image credit: NASA / ESA / JPL-Caltech / Conroy et al., doi: 10.1038/s41586-021-03385-7.
The new map reveals how a nearby dwarf galaxy called the Large Magellanic Cloud has sailed through the Milky Way’s halo like a ship through water, its gravity creating a wake in the stars behind it.
Though the inner portions of the halo have been mapped with a high level of accuracy, this is the first map to provide a similar picture of the halo’s outer regions, where the wake is found — about 200,000 to 325,000 light-years from the Galactic center.
“The robbing of a smaller galaxy’s energy is not only why the Large Magellanic Cloud is merging with the Milky Way but also why all galaxy mergers happen,” said Rohan Naidu, a graduate student at Harvard University.
“The wake in our map is a really neat confirmation that our basic picture for how galaxies merge is on point!”
In the study, Naidu and colleagues mapped the positions of 1,301 stars in the Milky Way’s halo.
The challenge arose in trying to measure the exact distance from Earth to a large portion of those stars: it’s often impossible to figure out if a star is faint and close by or bright and far away.
The astronomers used data from the Gaia satellite, which provides the location of many stars in the sky but cannot measure distances to the stars in the Milky Way’s outer regions.
After identifying stars most likely located in the halo, they looked for stars that belong to a class of giant stars that have a specific signature detectable by NEOWISE.
Knowing the basic properties of the selected stars enabled the team to figure out their distance from Earth and create the new map.
It charts a region starting about 200,000 light-years from the Milky Way’s center, or about where the Large Magellanic Cloud’s wake was predicted to begin, and extends about 125,000 light-years beyond that.
Using computer models, the researchers also found that the Large Magellanic Cloud is likely on its first orbit around the Milky Way.
If the smaller galaxy had already made multiple orbits, the shape and location of the wake would be significantly different from what has been observed.
The authors think that the Large Magellanic Cloud formed in the same environment as the Milky Way and the Andromeda Galaxy. Its next orbit will be much shorter due to its interaction with the Milky Way.
“Confirming our theoretical prediction with observational data tells us that our understanding of the interaction between these two galaxies is on the right track,” said Nicolás Garavito-Camargo, a doctoral student at the University of Arizona.
The new map may provide a test for different theories about the nature of dark matter, such as whether it consists of particles, like regular matter, and what the properties of those particles are.
“You can imagine that the wake behind a boat will be different if the boat is sailing through water or through honey,” said Harvard University’s Professor Charlie Conroy.
“In this case, the properties of the wake are determined by which dark matter theory we apply.”
“It’s a really special set of circumstances that came together to create this scenario that lets us test our dark matter theories,” said Dr. Gurtina Besla, an astronomer at the University of Arizona.
“But we can only realize that test with the combination of this new map and the dark matter simulations that we built.”
The results were published in the journal Nature.
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C. Conroy et al. 2021. All-sky dynamical response of the Galactic halo to the Large Magellanic Cloud. Nature 592: 534-536; doi: 10.1038/s41586-021-03385-7
