Huge star cluster discovered in neighbourhood of Milky Way
Saturday, January 14, 2006
Astronomers at Sloan Digital Sky Survey have announced the discovery of a huge star cluster, the Virgo super star cluster 10 kiloparsecs (32,600 light years) away from the solar system (roughly the same distance as the galactic centre), in the constellation of Virgo. Many of the stars have been known to astronomers for centuries, but only now have they realized the existence of the cluster.
In a presentation to the American Astronomical Society, Princeton University graduate student Mario Juric, the principal author of the findings, explained: "Some of the stars in this Milky Way companion have been seen with telescopes for centuries, but because the galaxy is so close, its stars are spread over a huge swath of the sky, and they always used to be lost in the sea of more numerous Milky Way stars. This galaxy is so big, we couldn't see it before."
The large overpopulation of stars was discovered by researchers analyzing SDSS-I and SDSS-II data. SDSS has, to date, imaged roughly one-quarter of the northern sky. The method used was the photometric parallax method. "We used the SDSS data to measure distances to 48 million stars and build a 3-d map of the Milky Way." explained Zeljko Ivezic of the University of Washington, a co-author of the study.
This discovery is one in a series of such discoveries made within the last decade by a new generation of sky surveys. The Saggitarius dwarf galaxy was discovered in 1994 using photographic sky images. Since then, analysis of photographs by large digital cameras has identified several streams and clumps of stars, some of which (astronomers believe) are Milky Way companions and others of which are shreds of the Saggitarius dwarf or other dwarf galaxies that are also dissolving into the Milky Way. An earlier SDSS discovery was the Canis Major Dwarf Galaxy. "With so much irregular structure in the outer Galaxy, it looks as though the Milky Way is still growing, by cannibalizing smaller galaxies that fall into it," said Juric.
The first indication that the Virgo super star cluster existed was in 2001, when the Quasar Equatorial Survey Team (QUEST) survey, using a 1-metre telescope in Llano del Hato National Astronomical Observatory in Venezuela, found a clump of five RR Lyrae variable stars, which astronomers speculated might belong to a small galaxy being canibalized by the Milky Way. Kathy Vivas of the Centro de Investigaciones de Astronomia in Venezuela, the author of the QUEST paper on the discovery, commented that "In light of the new SDSS results, it appears that the stellar stream we detected is itself part of the larger structure identified by Juric and collaborators."
In a letter to the Astrophysical Journal the QUEST has presented further evidence for Vivas' interpretation, by measuring the motions and chemical compositions of stars in the region.
- Juric, et al.. "The Sloan Digital Sky Survey Reveals A New Milky Way Neighbor" — , January 9, 2006
- Garyh S. Ruderman. Huge Star Cluster Discovered in Neighborhood of Milky Way <broken link> — , January 11, 2006
- "Discovery of possible dwarf galaxy announced" — , January 11, 2006
- "Huge star cluster discovered in neighborhood of Milky Way" — , January 11, 2006
- "The Sloan Digital Sky Survey Reveals A New Milky Way Neighbor" — , January 11, 2006
- "Gigantic Galactic Companion Discovered" — , January 11, 2006
- Sonia Duffau, Robert Zinn, A. Katherina Vivas, Giovanni Carraro, René A. Méndez, Rebeccah Winnick, and Carme Gallart; 2005 December 29. QUEST RR Lyrae Variables: The New Virgo Stellar Stream (open access) (subscription), Astrophysical Journal.