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Most Ambitious Star Survey Ever is Met with "RAVE" Reviews

02/14/2006 04:55PM

Most Ambitious Star Survey Ever is Met with "RAVE" Reviews
First results from the Radial Velocity Experiment (RAVE) project, an ambitious all-sky spectroscopic survey aimed at measuring the speed, temperature, surface gravity, and composition of up to a million stars, confirm that dark matter dominates the total mass of our home galaxy, the Milky Way. The full survey promises to yield a new, detailed understanding of the origins of the galaxy, according to team members at Johns Hopkins University.

The RAVE team is using the "six-degree field" multi-object spectrograph on the 1.2-m UK Schmidt Telescope at the Anglo-Australian Observatory, located at Siding Spring Observatory in New South Wales, Australia. The instrument is capable of obtaining spectroscopic information for as many as 150 stars at once, said Rosemary Wyse, a professor of Physics and Astronomy at Johns Hopkins. The RAVE team includes members from Australia, Germany, Canada, the Netherlands, the United Kingdom, Slovenia, Italy, Switzerland, France, and the United States.

"One important early application of RAVE aims to measure just how much stuff there is in our Milky Way galaxy — the collection of stars, gas, and dark matter that is the home of our Sun," Wyse said.

"Newton's Law of Gravity allows us to figure out from the orbital motions of stars how much mass is holding them together. Faster motions need more mass. We know from analyzing the motions in other galaxies that there is a lot more mass than we can see and this dark matter appears to dominate. But we are not sure exactly how much dark matter is needed in our own galaxy, and we don't know what the dark matter is made up of. That information is important, and the RAVE survey is going to help us answer some of those questions."

Greg Ruchti, a graduate student at Johns Hopkins notes that the project "needs large samples of very fast stars, and the unprecedented scope of the survey is ideal to find these rare objects."

"With more data and more modeling, the RAVE team plans to ascertain the Milky Way's overall mass, which at present is poorly understood," Wyse said. The team has what it considers a "better approach" to the problem: a model that makes very definite predictions about the way mass varies as a function of distance from the center of the Milky Way. If the team adopts this model, it can then estimate the overall mass from just the local "Escape Velocity," Wyse said.

Escape Velocity is the speed at which a star would have to be moving to leave the galaxy. The value of this special speed depends on the mass of the galaxy -- the higher the mass, the higher the speed necessary to escape. Thus, researchers can estimate the weight of the Milky Way galaxy by measuring how fast objects must move to leave it, Wyse said.

Current RAVE limits show that stars would need to move faster than around 500 km/second to escape, more than twice as fast as the Sun is moving around the galactic center.

"Some groups believe that our neighbor, the Andromeda Galaxy — also known as M31 — is the most massive galaxy in our local group. But we suspect from our early results that our Milky Way is actually the local heavyweight," said Martin Smith of the University of Groningen in the Netherlands. "We are, with RAVE, on the verge of an answer."

"RAVE will run for several more years, and the full RAVE survey will provide a vast resource of stellar motions and chemical abundances, allowing us to answer fundamental questions about the formation and evolution of our galaxy," said Matthias Steinmetz, director of the Astrophysical Institute Potsdam, and leader of the RAVE collaboration.

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