ANALYSIS: The Fossils of Galactic Cannibalism
The favored explanation seems to be that of a "galactic shell" of stars left over from the "eating" of a smaller galaxy by the Milky Way in the distant past. Such shells are visible in the halos of other galaxies, and the Milky Way is known to have streams of stars that hang together after a smaller galaxy has been ripped apart. Since these halo stars are so far out, they may trace an even more distant past. Other explanations include a new stream of stars, stars that have formed in the halo itself, or a rotation of the halo that has not been accounted for.
This pilot study was only done, however, with only 13 stars over a very small region of sky, whereas the halo envelopes us in every direction. Naturally, the team intend to continue looking for highly accurate data sets of other regions of the sky with similarly long time periods of monitoring. The proper motion study of Andromeda just happened to be a good place to start, since the data already existed to look for the tiny proper motions of distant stars.
As well as telling the merger history of our Galaxy, these stars are also in the further region of the halo that becomes dominated by dark matter. A close study of their motion might then reveal the true shape of the dark matter halo surrounding us, giving us more insight into that mysterious "stuff" that is so prevalent in the Universe.
ANALYSIS: Hubble Spies Aftermath of Galactic Cannibalism
What other gems lurk in the data archives of Hubble and the other large telescopes that have yet to be discovered?
Image: Artist's conception of the Milky Way disk, stellar halo, and the newly discovered shell of stars in the direction of the Andromeda Galaxy. Hubble Telescope quite obviously not to scale. Credit: NASA, ESA, A. Feild (STScI), A. Deason and P. Guhathakurta (University of California, Santa Cruz), and R. van der Marel, T. Sohn, and T. Brown (STScI).
This work is being published in Astrophysical Journal, and a preprint is available at arXiv.org.