An international team of astronomers has found evidence that a number of stars located in the distant Galactic halo surrounding the Milky Way were ejected following either a collision or close pass with a smaller dwarf galaxy.
Such stars are grouped together in giant formations that orbit the center of the Milky Way, above and below the flat disk of our galaxy.
The majority of stars in the Milky Way rotate in a massive disk around the galactic center, where lies a supermassive black hole called Sagittarius A. Yet a significant number of stars orbit beyond the outer edges of this disk, forming the Galactic halo.
Researchers previously thought that these halo stars were stolen from smaller dwarf galaxies as they passed close to, or merged with our galaxy. However, new evidence suggests that some of the halo stars were actually created in the outer regions of the Milky Way, and then evicted to the far edges.
“This phenomenon is called galactic eviction,” said Judy Cohen, a professor at California Institute of Technology in the US and co-author of the study.
According to Cohen, dwarf galaxies create oscillations or waves in the Milky Way’s structure once they interact or pass through our galaxy. These fluctuations would be powerful enough to cast out groups of stars from the disk and throw them out to orbit the Galactic halo.
“These oscillations can be compared to sound waves in a musical instrument,” said lead author Maria Bergemann of the Max Planck Institute for Astronomy in Heidelberg, Germany.
Bergemann and her team focused on 14 stars located in two different halo populations, the Triangulum- Andromeda, and the A13 stellar overdensities. The star structures lie on opposite sides of the galactic disk, positioned roughly 14 thousand light years above and below the disk.
The study was published in the journal, Nature.
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