By Ron Cowen
From Minneapolis, at a meeting of the American Astronomical Society
Visible even to the naked eye, the starlit, spiral disk of the Andromeda galaxy stretches across a patch of sky as wide as the full moon. A new investigation of Earth’s next-door-neighbor galaxy reveals that it looms even larger. Using a spectrograph on the Keck II Telescope atop Hawaii’s Mauna Kea, astronomers have found that the diameter of Andromeda’s disk spans about 220,000 light-years. That’s three times the size of previous estimates.
A team led by Scott Chapman of the California Institute of Technology in Pasadena and Rodrigo Ibata of the Strasbourg Observatory in France measured the motion toward or away from Earth of some 3,000 relatively old, outlier stars in Andromeda. These outliers were once thought to lie in Andromeda’s halo, a spherical distribution of the galaxy’s stars that loops around the disk. The analysis shows that the Andromeda outliers are instead fringe residents of the disk and therefore expand the boundary of Andromeda’s disk, says Chapman.
“Finding [such outliers] over the whole of Andromeda establishes the suspicion that the disk is extended,” comments R. Michael Rich of the University of California, Los Angeles. He and his colleagues had previously found hints of an extended Andromeda disk using the Keck II Telescope and a smaller telescope at the Kitt Peak National Observatory in Tucson.
