Magellanic Clouds May Be Just Passing Through (Forwarded)

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For Release: 9:00 a.m. PST, Tuesday, January 9, 2007

Release No.: 07-02

Magellanic Clouds May Be Just Passing Through

Seattle, WA -- The Large Magellanic Cloud (LMC) and Small Magellanic Cloud
(SMC) are two of the Milky Way's closest neighboring galaxies. Both are
visible only in the southern hemisphere. By studying their orbits,
astronomers can learn about both the histories of the Clouds and the
structure of the Milky Way (from its influence on the Clouds' motions).

Astronomers Nitya Kallivayalil and Charles Alcock (Harvard-Smithsonian
Center for Astrophysics) and Roeland van der Marel (Space Telescope Science
Institute) have made the most accurate measurements to date of the
three-dimensional velocities through space of the LMC and SMC. Their
surprising results hold profound implications for both the Milky Way and its
companions.

"We found that the velocities of the LMC and SMC are unexpectedly large --
almost twice those previously thought," says Kallivayalil.

These findings were presented today in a press conference at the 209th
meeting of the American Astronomical Society.

The radial velocities (motion along the line of sight) for both Clouds are
well known and relatively easy to measure.

Much more difficult to measure is the proper motion (motion across the sky),
requiring extraordinary precision over the course of several years. Both
proper motion and line-of-sight motion must be known to calculate the true
3-d velocity.

By making two sets of observations two years apart with NASA's Hubble Space
Telescope, Kallivayalil and her colleagues calculated accurate proper
motions for the LMC and SMC. By combining proper motions and radial
velocities, they found that the LMC speeds through space at 378 km/sec (235
miles/sec) while the SMC has a speed of 302 km/sec (188 miles/sec).

There are two possible explanations for these high speeds:

1) The mass extent of the Milky Way is larger than previously thought. If
the Clouds are gravitationally bound to the Milky Way, then the Milky Way
must be much more massive than previous data suggested. The excess mass
would pull on the Clouds, keeping them "close at hand."

2) The Magellanic Clouds are not gravitationally bound to the Milky Way. If
previous calculations of the Milky Way's mass are accurate, then the Galaxy
is not massive enough to hold onto its companions. In a few billion years,
they will escape from the Milky Way.

"The Magellanic Clouds may not be true companions of the Milky Way,"
explains Kallivayalil. "Perhaps they are travelers just passing through the
neighborhood."

The velocities of the Magellanic Clouds relative to each other also are
surprisingly high. This suggests that the Magellanic Clouds may be
coincidental companions and are not gravitationally bound to each other.
Alternatively, their high velocities may explain why these two galaxies, if
bound, did not merge with each other long ago.

Future measurements of the Magellanic Stream -- a long streamer of hydrogen
gas trailing behind the Clouds -- may clarify the previous paths of the
Clouds and their relationships with each other and with the Milky Way.

"Regardless of what future work finds, our study shows that we need to
reassess the orbital histories of the Clouds," says Kallivayalil.

The Hubble Space Telescope is a project of international cooperation between
NASA and the European Space Agency. The Space Telescope Science Institute,
Baltimore, conducts Hubble science operations. The Institute is operated for
NASA by the Association of Universities for Research in Astronomy, Inc.,
Washington.

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

Note to editors: A high-resolution image to accompany this release is online
at
http://www.cfa.harvard.edu/press/pr0702image.html