Why do bulges appear in the Milky Way?

A new simulation running on the world’s most powerful supercomputer focused on astronomy has developed a testable hypothesis to explain how the Milky Way’s stripe looked. This scenario will be compared to data from past, current and future space telescopes to help shed light on the evolution of the Milky Way.

A Milky Way-like galaxy simulated by the ATERUI II supercomputer. Stars are clustered in a band near the center of the galaxy. Image Credit: Junichi Baba, Hirotaka Nakayama, 4D2U Project, NAOJ

Astronomy reveals the structure of the Milky Way gradually. Researchers know it is a disk galaxy with one or four armed spirals attached to a straight rod in the center.

Currently, researchers know that the inner section of the bar has a “nuclear bulge,” which is disc-shaped and located in the center of the galaxy, as well as a “peanut-shaped bulge,” or an area where the ribbon is denser and protrudes above and below the central plane of the galaxy. Some other galaxies, but not all, display some of the same bulges of both types.

Astronomers have been puzzled over how these bulges form. ATERUI II, the world’s most powerful astronomy-focused supercomputer, used by a team at National Astronomical Observatory of Japan (NAOJ) under the direction of Junichi Baba to explore a single scenario of a Milky Way-like galaxy.

The cluster simulation includes galactic gas and stars, making it the most comprehensive and realistic to date. It also studies the demise of stars through supernovae and the creation of new stars from gas.

The development of the rod facilitates the flow of gas to the center of the galaxy, where it sparks the birth of new stars. Therefore, it is reasonable to theorize that the galactic nuclear horn is composed of newly formed stars.

However, because the streak is so efficient at guiding the gas toward the center, simulations indicate that there are hardly any new stars in the streak outside of the nuclear bulge. This proves that the peanut-shaped bulge in the bar does not form due to excessive gas consumption.

The team eventually discovered that some stars can be pushed into orbits that carry them above and below the mean through gravitational interactions.

The simulation’s ability to create a testable scenario is the most amazing feature. All the stars in the peanut-shaped bulge must have formed before the keel because it doesn’t pick up any new stars. The bar also directs gas to the center, where it forms many young stars.

Nuclear bulge stars will be born almost exclusively after the evolution of the streak. This means that there will be a distinct age difference between stars in the nuclear bulge and those in the peanut-shaped bulge. This break occurred when the tape first appeared.

The researchers will be able to confirm the motions and ages of the stars and test this hypothesis using data from the European Space Agency’s Gaia probe and the upcoming Japanese satellite, JASMINE.

Astronomers can determine the age of the Milky Way’s bar if they can distinguish between the ages of peanut-shaped stars and nuclear bulges. This will show that excessive gas consumption is not the cause of bloating.

Journal reference:

child, c. and others. (2022) Age distribution of stars in square/peanut/X-shaped bulges formed without torsion of a rod. Get access arrow. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/stac598.

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