The Milky Method is historic, and its first stars shaped shortly after the Huge Bang. It has been by lots in its lengthy life, together with merging with different galaxies and absorbing smaller ones. Our observations of the cosmos affirm that, the place we see a number of examples of galaxies merging and interacting. Piecing collectively the Milky Method’s historical past of mergers and different occasions is vital to understanding our galaxy, and by extension, different galaxies.
Not all the things concerning the Milky Method (MW) is clearcut. Even with the entire scientific progress in astronomy, particularly within the final couple many years, a few of its options are nonetheless puzzling. Certainly one of its puzzles considerations its disk. It has two separate disk buildings that rotate at totally different speeds.
The majority of the galaxy’s stars are in its spiral disk, or skinny disk, however a a lot bigger and extra sparse disk surrounds that disk as a stellar halo, or thick disk. These halo stars initially shaped in several galaxies, however had been absorbed by the MW throughout mergers. The ESA’s Gaia mission noticed billions of stars within the MW and located that the thick disk rotates very slowly in comparison with the skinny disk.
*This schematic exhibits the principle options of the Milky Method, together with each of its disks. Picture Credit score: By RJHall at English Wikipedia, CC BY-SA 3.0,
Astronomers have labored exhausting to attempt to perceive why. Because the thick disk stars are older, metal-poor, and comply with eccentric or extra random, chaotic orbits, they’re proof of a previous merger. To dig into the connection between the sluggish rotation of the sparsely populated halo, and historic mergers, a workforce of researchers used simulations. They offered their outcomes on the Royal Astronomical Society’s Nationwide Astronomy Assembly in Birmingham.
“As a result of we reside contained in the Milky Method, we are able to examine it in additional element than some other galaxy, which makes it a key testbed for understanding galaxies extra broadly.” – Kirill Batrakov, Durham College.
The researchers, led by Kirill Batrakov, an astronomer from Durham College, used the Auriga simulations. The Auriga simulations are a set of cosmological zoom simulations geared toward understanding how galaxies type. The simulations bear in mind black holes, supernovae, darkish matter, and gravity, and start shortly after the Huge Bang. Auriga simulations create sequences of scientific snapshots that reveal how occasions in a galaxy’s historical past created the buildings we observe immediately.
The researchers simulated 25 galaxies much like the Milky Method, and let the simulations run for billions of years.
“Gaia observations have revealed that the Milky Method (MW) stellar halo has a weak internet rotation of roughly 10-20 km/s, but the origin of this rotation stays unexplained,” the researchers write in their abstract. “Motivated by this discovering, I investigated the rotation of stellar haloes over a redshift vary of 0-2.5 in Auriga – a set of magnetohydrodynamical cosmological zoom-in simulations of MW-like galaxies.”
That redshift vary covers roughly the final 11 billion years, so every simulation started with a MW-like galaxy about 2 to three billion years after the Huge Bang. That vary consists of the period of peak cosmic star formation, the “cosmic midday” round z ≈ 1-3. It additionally consists of many of the main mergers these galaxies expertise
The simulations that recreated the MW’s slowly rotating outer halo shared three issues in widespread: they shaped earlier; they underwent a merger just like the MW’s Gaia-Enceladus-Sausage merger; they usually skilled a disk-flip occasion.
“We already know that the Milky Method had an enormous head-on collision prior to now with a galaxy generally known as Gaia-Sausage-Enceladus (usually merely known as the Gaia Sausage). So, we predict that the Milky Method disc seemingly flipped prior to now,” defined Batrakov, the lead researcher on the undertaking.
“A disc flip additionally means many of the Milky Method’s stars as soon as moved on very totally different trajectories than they do immediately – presumably even our personal Solar, that means our ‘steady’ spot within the galaxy may not have been so steady for the Photo voltaic System’s complete lifetime,” Batrakov stated.
We’re contained in the Milky Method, in order that creates some difficulties in learning it. However our place additionally works in our favour. The ESA’s Gaia mission has modified our understanding of the Milky Method by measuring billions of MW stars, one thing not potential to the identical diploma in different galaxies.
“Discovering that its disc flipped provides a brand new chapter to that story, one we should account for when putting the Milky Method in a broader context of different galaxies.” Kirill Batrakov, Durham College.
“As a result of we reside contained in the Milky Method, we are able to examine it in additional element than some other galaxy, which makes it a key testbed for understanding galaxies extra broadly,” Batrakov stated. “Discovering that its disc flipped provides a brand new chapter to that story, one we should account for when putting the Milky Method in a broader context of different galaxies. What excites me probably the most is that this complicated historical past could be reconstructed simply from present-day observations.”
The outcomes additionally say one thing concerning the MW’s darkish matter halo. “These outcomes recommend that the MW seemingly underwent a disc flip in its formation historical past, and that its DM halo seemingly rotates comparatively slowly,” the authors write.
This means a connection between the 2. The stellar halo might have developed alongside the DM halo because the MW continued accreting smaller galaxies all through its historical past, simply because it’s doing now with the Magellanic Clouds and the Sagittarius Dwarf Spheroidal Galaxy.
Different analysis additionally helps the disk flip state of affairs. A 2025 paper in Astronomy and Astrophysics targeted on the Milky Method’s darkish matter halo and used Gaia knowledge to grasp it. “We discover tentative proof that the Milky Method DM halo is twisted, per alignment with the disc within the inside r ≲ 20 kpc, and turns into vertically oriented within the outer areas, per the prediction of disc flip state of affairs,” they write.
The truth of residing inside an enormous galaxy that has developed over billions of years, comprises a whole bunch of billions of stars, has undergone mergers, and has flipped its disk, would not contact our on a regular basis lives. However it’s actuality, and mental curiosity drives us to grasp it. Anybody who enjoys the scientific journey of understanding about our residence galaxy ought to tip their hat to the European Area Company’s Gaia mission, which is revolutionizing our understanding of the Milky Method.
