Some of the Milky Way’s oldest stars have been spotted in a surprising place — the disk the youngest region of the galaxy. Computer 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.  Isaiah Santistevan, a doctoral candidate in our department is the lead author of a new study examining the simulation results, which is available on arXiv.org.

The oldest stars in the Milky Way are "metal-poor", with low amounts of carbon, oxygen, iron and other elements heavier than helium that only form in stars. That’s why astronomers think "metal-poor" stars are some of the first stars born after the Big Bang, at least 11 billion years ago, long before our sun formed about 4.5 to 5 billion years ago.

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 an 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 found.