Scientists "RAVE-ing" about Most Ambitious Star Survey Ever (Forwarded)

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Embargoed for Release on January 11, 2006 at 9:20 A.M. EST

Scientists "RAVE-ing" about Most Ambitious Star Survey Ever

An international team of astronomers today announced the first results
from the Radial Velocity Experiment, an ambitious all-sky spectroscopic
survey aimed at measuring the speed, temperature, surface gravity and
composition of up to a million stars passing near the sun.

Those first results from the project, known for short as RAVE, confirm
that dark matter dominates the total mass of our home galaxy, the Milky
Way, team members at The Johns Hopkins University and elsewhere said.
The full survey promises to yield a new, detailed understanding of the
origins of the galaxy, they said.

The results were released at the American Astronomical Society's 207th
meeting in Washington, D.C.

The 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 in the Henry A.
Rowland Department of Physics and Astronomy in Johns Hopkins' Krieger
School of Arts and Sciences and a member of the RAVE team. RAVE includes
members from the United States, Germany, Australia, Canada, the
Netherlands, the United Kingdom, Slovenia, Italy, Switzerland and France.

"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 in physics and astronomy at Johns
Hopkins who also is a member of the RAVE team, 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. I'm really excited about
being part of the RAVE team."

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. At that escape speed, it would take
less than eight seconds to travel from Baltimore to Los Angeles.

"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."

Funding for RAVE is provided by the National Science Foundation, for
Johns Hopkins, and by the national research councils of other team
members' countries as well as by private sources.

"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.

A larger version of the image above is available online at
http://www.jhu.edu/news/home06/jan06...s/mw_orbit.jpg (66KB)

Digital photos of Wyse and Ruchti also are available. Contact Lisa De
Nike at 443-287-9960.

IMAGE CAPTION:

Lund map of the Milky Way galaxy

Schematic orbits of stars moving quickly past the Sun, indicated on the
Lund map of the Milky Way galaxy (copyright Lund Observatory, used with
permission). The approximate distance of the Sun from the galactic
center is the intersection of the three curves. Each curve indicates the
orbit of a high-velocity star, with the arrow at the intersection
indicating that star's velocity as it passes the sun. Note that the
stars are moving at speeds less than the escape velocity.