Milky Way stars kicked out by invading galaxies spotted

London: An international team of astronomers has discovered evidence to show that some stars located in the Galactic halo surrounding the Milky Way were original residents of our galaxy but were kicked out from their birthplace by some invading galaxies. The surprising discovery about the birthplace of groups of stars located in the halo of our Milky Way galaxy was detailed in the journal Nature.

These halo stars are grouped together in giant structures that orbit the centre of our galaxy, above and below the flat disk of the Milky Way. Researchers earlier thought they may have formed from debris left behind by smaller galaxies that invaded the Milky Way in the past. The new study showed that some of these halo structures actually originated from the Milky Way's disk itself, but were kicked out. "This phenomenon is called galactic eviction," said co-author Judy Cohen, Professor at California Institute of Technology (Caltech) in the US. 

"These structures are pushed off the plane of the Milky Way when a massive dwarf galaxy passes through the galactic disk. This passage causes oscillations, or waves, that eject stars from the disk, either above or below it depending on the direction that the perturbing mass is moving," Cohen said.  "The oscillations can be compared to sound waves in a musical instrument," said lead author Maria Bergemann of Max Planck Institute for Astronomy (MPIA) in Germany.

"We call this 'ringing' in the Milky Way galaxy 'galactoseismology,' which has been predicted theoretically decades ago. We now have the clearest evidence for these oscillations in our galaxy's disk obtained so far!" Bergemann said. Bergemann's team presented detailed chemical abundance patterns of these halo stars using the W. M. Keck Observatory on Maunakea, Hawaii. "Once we know what the stars are made of, we can immediately link them to their parent populations," said Bergemann.

The scientists investigated 14 stars located in two different halo structures -- the Triangulum-Andromeda (Tri-And) and the A13 stellar overdensities. These two structures lie on opposite sides of the Milky Way disk -- about 14,000 light years above and below the Galactic plane.

"We showed that it may be fairly common for groups of stars in the disk to be relocated to more distant realms within the Milky Way -- having been 'kicked out' by an invading satellite galaxy," said co-author Allyson Sheffield of LaGuardia Community College in New York.