Drama Around the Galaxy

February 27, 2018

An international team of astronomers combine disparate lines of evidence to piece together a dramatic revision to our understanding of the recent history of our home galaxy - the Milky Way. An article published in Nature on 2/26/18 provides the final confirmation.
The Milky Way is the humanity’s home galaxy and in some ways we know it very well: it is the only galaxy in the Universe whose stars we can see every (clear) night with the naked eye. Astronomers classify the Milky Way as a fairly average, large spiral galaxy with the majority of its stars circling its center within a disk, and a dusting of stars beyond that orbiting in the Galactic halo.

Several years ago, our team decided to study a set of structures that took the form of partial rings lying well beyond what was then considered the edge of the disk. Allyson Sheffield (then at Columbia, now La Guardia College) led the first work to measure the speeds of stars in the most distant structures, known as the Triangulum-Andromeda Clouds - demonstrating that they formed coherent sequences in speed as well as space. Kathryn Johnston (Columbia) produced simulations that could explain the positions and speeds with the disruption of just one galaxy orbiting our own in the opposite sense to stars in the Galactic disk.
Slowly, the team together realized the larger implications of our findings. Adrian Price-Whelan (then a Columbia graduate student, now at Princeton) uncovered evidence by looking at the types of stars these structures were made up of that they did not result from invasion from outside the Galaxy but rather eviction from the disk itself!
Chervin Laporte (then at Columbia, now in Victoria) undertook a series of simulations of the impact of some of the still-existing satellite galaxies that are just now invading the Milky Way to see if this interpretation made sense (see image)

The final confirmation of eviction was published on 2/26/18 in the journal Nature. Maria Bergeman and Brani Sesar (MPIA, Heidelberg) have measured the compositions of several of the M giant stars that are members of the rings around our Galaxy. They find the stars are made of a mix of elements very similar to the Galactic disk and unlike the mix seen in stars in the halo of our Galaxy or in the still-invading satellites.
Conclusion - Cosmic space invaders and eviction in process!
In the end, our story did turn out to about invasion, but not in the sense we originally assumed - rather an invading galaxy evicting stars from our own disk to form part of our halo. And we have a front-row seat on the action!

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