Simulations Reveal Signs of Galaxy Mergers in Milky Way Disk

Image of computer simulations of galaxy history
Images from a computer simulation show a galaxy similar to the Milky Way early in its history (left) and today (right). (Courtesy Isaiah Santistevan)

Some of the Milky Way’s oldest stars have been spotted in a surprising place — the disk that is our galaxy’s youngest region. Supercomputer simulations of their orbits suggest these metal-poor stars came from a smaller galaxy that slammed into the Milky Way more than 7 billion years ago.

“Understanding why these stars are orbiting in the disk can tell us more about how the Milky Way formed,” said Isaiah Santistevan, a doctoral candidate in the UC Davis Department of Physics and Astronomy and lead author of a new study examining the simulation results. The study is available on arXiv.org.

The most primitive, oldest stars in the Milky Way are metal-poor, with low amounts of carbon, oxygen, iron and other metallic elements that only form inside stars. That’s why astronomers think metal-poor stars are some of the first stars born after the Big Bang. “Metal-poor stars formed at least 11 billion years ago, whereas a star like our sun formed maybe 4.5 to 5 billion years ago,” Santistevan said.

Most of the metal-poor stars in the Milky Way cluster in the galaxy’s oldest region, the outer halo. But a few travel an almost circular path in the flat plane that is the Milky Way's disk, following a prograde orbit similar to the sun’s.

To figure out how these stellar artifacts ended up in the disk, Santistevan and his collaborators analyzed realistic simulations of galaxies from the FIRE-2 (Feedback In Realistic Environments) project. The simulations include everything scientists know about how galaxies form and evolve.

According to the simulations, the majority of metal-poor stars in the disk formed in a different, much smaller galaxy that later merged with the Milky Way. The galactic collision not only left stellar artifacts scattered in the disk, it also brought in younger stars and dust that helped form the disk itself, Santistevan said. “This is really cool because when people think about the formation of disks in galaxies, they don’t often think of galaxy mergers as being responsible,” he said.

Becky Oskin, content strategist in the College of Letters and Science

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