The group of stars that lies in the Milky Way's halo has a surprising origin, according to a new study.
The halo stars are clustered together in massive structures orbiting the Milky Way's center, both over and under its flat disk. Astronomers previously held that they had originated from the debris left by smaller galaxies that had pervaded the Milky Way in the past.
However, a team of researchers led by the Max Planck Institute for Astronomy has made an interesting discovery about the origin of the Milky Way's halo stars. They found evidence that a few of the halo structures belonged to the disk of the Milky Way itself.
Galactic Eviction Creates Milky Way's Halo Stars
“This phenomenon is called galactic eviction,” said study coauthor Judy Cohen. “These structures are pushed off the plane of the Milky Way when a massive dwarf galaxy passes through the galactic disk."
Cohen added that the passage results in waves or oscillations that evict stars from the galactic disk, either above or under. It depends on the moving direction of the unsettled mass.
Maria Bergemann, the lead author of the study, added that the oscillations are comparable to the sound waves produced by musical instruments. The oscillations also referred to as ringing in the Milky Way galaxy is known as galactoseismology. This was theoretically predicted decades ago and now, scientists obtained their clearest evidence with the help of the W.M. Keck Observatory in Hawaii's Maunakea.
The team analyzed the halo stars and their chemical abundance patterns, which helped in identifying the star's parent population. Different parent populations, like the Milky Way halo or disk, globular clusters, or dwarf satellite galaxies usually have extremely different compositions chemically. Once researchers know a star's composition, they can immediately relate them to a parent population.
The research team studied 14 stars that lie in two different structures of halo: the A13 stellar overdensities, and the Triangulum-Andromeda. They lie on the Milky Way disk's opposite sides around 14,000 light years over and under it.
Experts were surprised to discover that the chemical composition of the stars located in the two cosmic structures is almost similar, both between and within these groups. They are also observed to closely resemble the abundance patterns of the outer disk stars of the Milky Way, providing strong proof that the stars in the halo have evolved from the galaxy's thin disk.
The new findings published in the journal Nature excite the researchers because they suggest that the galaxy's disk and dynamics are more intricate than previously held. The team now wants to study the spectra of more stars in the A13 and Tri-And overdensities in addition to the other stellar structure stars located farther from the disk.
The scientists also plan on determining the ages and masses of the halo stars to understand the timeline of the galactic eviction.