Huge star cluster discovered in neighbourhood of Milky Way

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Saturday, January 14, 2006

Map of the Virgo super star cluster
M. Juric/SDSS-II Collaboration

This is a map, by Mario Juric of Princeton University based upon the forthcoming SDSS-II Data Release 5, of the Virgo super star cluster, viewed from far away and from above the galactic plane. It shows the counts of faint blue stars, from a narrow magnitude and color range, that are 10 kiloparsec away from Earth.

The map is shown in the Lambert projection of galactic coordinates: radial rays are lines of constant longitude, circles are lines of constant latitude, the North Galactic Pole is in the center, and the Galactic Centre is towards the left. The counts are shown on a logarithmic stretch, with a dynamic range of 10 increasing from blue to red.

The dotted line through the middle of the map shows the plane of the debris from the Saggitarius dwarf galaxy that is being cannibalized by the Milky Way. This debris is approximately four times further away from Earth than the Virgo super star cluster is. Astronomers have hypothesised that the two may be related.

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.

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