Milky way stars kicked out by invading galaxy

The outer “halo” region of the Milky Way contains the vast majority of the mass of the Galaxy. Unlike the Galaxy’s spiral arms, which contain bright stars, the halo is mostly dark, but they do contain some globular clusters of stars, some of the oldest stars in the Milky Way.

These stars are generally thought to be ghosts of dwarf galaxies past, long ago torn into shreds after encounters with our more massive galaxy. Now, new research involving ASTRO 3D researcher Dr Luca Casagrande, shows that some of these stars might not be dwarf remnants at all – they might have come from the Milky Way’s own disk. The question then becomes: how did they travel all the way from the disk out into the stellar halo?

The researchers found the chemical composition of two outlying groups of stars, A13 and Triangulum-Andromeda, which are about 14,000 light years above and below the plane of the Milky Way, closely matched the stars in our galaxy.

“We think these stars were evicted through a tidal interaction of the Milky Way and a dwarf galaxy,” said Dr Casagrande, an ARC Future Fellow at the ANU Research School of Astronomy and Astrophysics. Tidal interactions between galaxies involve the gravitational field of each galaxy distorting the other –such interactions can change, sometimes dramatically, the form and structure of the galaxies involved.

“These findings are very exciting, as they indicate that the Milky Way Galaxy’s disk as a whole can oscillate because of tidal interaction and it’s dynamics are significantly more complex than previously thought.”

Dr Casagrande measured the temperature of the stars involved in the study, which enabled the team’s analysis of stellar distances and chemical compositions. He used the infrared flux method, a technique regarded as one of the most reliable to measure temperature in stars.

ANU and research institutions in Germany, the United States, Spain, the United Kingdom and South Korea supported the study, which was led by the Max Planck Institute for Astronomy in Germany. The research was published in Nature.