New Stellar Stream Found in Milky Way

Yale astronomers have reported the discovery of a stream of stars believed to be the remnant of an ancient star cluster slowly being ingested by our Milky Way Galaxy.

This image shows the southern sky as mapped by the Sloan Digital Sky Survey. The newly discovered stellar stream is designated as the Triangulum stream (SDSS DR 8 / Ana Bonaca et al)

The astronomers found evidence for the new stream by analyzing stellar density maps based on data from the Sloan Digital Sky Survey Data Release 8.

The stream, named the Triangulum stream, is located some 84,800 light-years away in the direction of the Andromeda (M31) and Triangulum (M33) galaxies. It spans 0.2 degrees by 12 degrees on the sky, or 75 by 5,500 parsecs in physical units, and consists primarily of old, metal-poor stars.

Although researchers have previously found evidence of the Milky Way eating up dwarf galaxies, the newly found stellar stream is believed to be the remnant of a star cluster rather than of a larger galaxy, because the stream is very narrow.

“The Milky Way is constantly gobbling up small galaxies and star clusters,” said Ana Bonaca, a graduate student at Yale University and lead author of a study to be published in the Astrophysical Journal Letters (arXiv.org version). “The more powerful gravity of our Milky Way pulls these objects apart and their stars then become part of the Milky Way itself.”

“Our discovery is more of a light snack than a big meal for the Milky Way,” explained study co-author Prof Marla Geha. “Studying this digestion process in detail is important because it gives us new insight into how all galaxies form and evolve.”

The Triangulum stream is the first of its kind found in the southern Galactic sky, a region that has been hard to examine due to a relative lack of deep-sky imaging there. According to the astronomers, the discovery will help reconstruct how the Milky Way’s mass is distributed, further revealing its dynamic structure.

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Bibliographic information: Ana Bonaca et al. 2012. A Cold Milky Way Stellar Stream in the Direction of Triangulum. To be published in Astrophysical Journal Letters; arXiv: 1209.5391