An international team of astronomers last week
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 Johns
Hopkins 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 Jan. 11 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-meter 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
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Greg Ruchti, a graduate student in physics and
astronomy at Johns Hopkins who also is a member of the RAVE
team, noted 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 500 kilometers or so per 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.
Team member Martin Smith, of the University of
Groningen in the Netherlands, said, "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. 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.
Matthias Steinmetz, director of the Astrophysical
Institute Potsdam and leader of the RAVE collaboration,
said, "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."