If you look at the Milky Way from “above,” it almost looks like a cat’s eye: a circle of spiral arms with an oval “iris” in the middle. That iris — a starry bar that connects the spiral arms — has two strange bulges: one peanut-shaped and one disc-shaped, neither conforming to the otherwise pleasingly geometric galaxy. These same deformations appear in some other galaxies. Now, researchers from the National Astronomical Observatory of Japan (NAOJ) have applied supercomputing to investigate why and how these bulges form in galaxies.
To answer this question, the researchers ran “the most complete and accurate to date” simulation of the formation of a Milky Way-like galaxy — a simulation that included not only the stars, but the gases between them. To run that simulation, they used ATERUI II, NAOJ’s in-house supercomputer, and the most powerful supercomputer in the world dedicated to astronomy. ATERUI II, a Cray system that launched in mid-2018, consists of 1,005 nodes powered by Intel Xeon Gold 6148 CPUs. The system has a total of 385.9TB of memory and delivers 2.09 Linpack petaflops, placing it 349th on the most recent Top500 list. It is complemented by 6.5PB of storage.
That bar that connects the arms channels gases into the middle of the galaxy, helping to form new stars — but the forming stars are constrained to the disc-type bulge at the center of the galaxy, and so do not explain the peanut-shaped bulge in the bar. Instead, the researchers found that the peanut-shaped bulge is explained by gravitational interactions that pushed stars out of the normal plane of the galaxy — interactions that predate the formation of the bar itself, whereas the stars housed in the disc-shaped bulge likely formed after the bar appeared.
The researchers also released a beautiful video of the simulation, embedded below.
The researchers at NAOJ are excited about the age gap aspect of the discovery, as it creates an easily testable hypothesis: if astronomers find that the “peanut” stars are dramatically older than the “disc” stars, then the hypothesis is correct. So, next, the team plans to use data from the European Space Agency’s Gaia probe and Japan’s forthcoming JASMINE satellite to collect this data.
To learn more about this research, read the paper, which was published in Monthly Notices of the Royal Astronomical Society as “Age distribution of stars in boxy/peanut/X-shaped bulges formed without bar buckling.” The paper was written by Junichi Baba, Daisuke Kawata and Ralph Schönrich.