The Milky Way's Galactic Bar Punches Holes in Stellar Streams

September 1, 2017

Illustration of our Milky Way galaxy viewed from above (including the central galactic bar), overlayed with the projection of the Palomar 5 stream (orange) which has visible holes due to previous interactions with the Galactic bar. Credit: NASA/JPL-Caltech, Pearson, Price-Whelan

Our own Galaxy, the Milky Way, hosts a so-called Galactic bar, which is a collection of billions of stars coherently rotating in the plane of our Galaxy. In a recent paper published in Nature Astronomy , Columbia graduate student Sarah Pearson and co-authors Dr. Adrian Price-Whelan and Prof. Kathryn V. Johnston discovered that the galactic bar, can punch holes in the Palomar 5 stellar stream. Palomar 5 is an old cluster of stars orbiting our Galaxy, while disrupting into thin leading and trailing stellar arms due to the Milky Way’s gravitational pull. Despite the present day location of the Palomar 5 cluster being ~52.000 light years above the Galactic plane, the authors found that a previous encounter between the stream and the Galactic bar could explain why the leading arm of Palomar 5 appears to be a lot shorter than the trailing arm, which have otherwise been predicted to be of similar lengths.

The discovery that the bar can punch holes in stellar streams that grow over time has important implications for cosmology. From our current understanding of our Universe, a Galaxy, like the one we live in, should be filled with dark matter clumps of various sizes. However, these dark matter clumps are extremely difficult to find as they are solely made up of, the yet to be detected, dark matter. One proposed method to detect the dark matter clumps, is to search for them through disturbances in the stellar structure of stellar streams, as the dark matter clumps should create holes if they pass through or close by the stellar streams. However, Pearson and her co-authors demonstrated that the Galactic bar can create holes of very similar appearance to those created by dark matter clumps and the authors caution that astronomers should not necessarily interpret the holes in stellar streams as evidence of the existence of dark matter clumps.

Additionally, the fact that the Galactic bar can punch holes that grow and have locations along stellar streams dependent on the Galactic bar orientation, mass and rotational speed, provides an intriguing methodology for studying our own Milky Way’s Galactic bar in more detail.

Link to article in Nature Astronomy >>

Link to Nature Astronomy News & Views article >>

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