CfA Release No: 2007-22

For Release: Monday, Sept. 17, 2007

Note to editors: A high-resolution photo to accompany this release is available
online at http://www.cfa.harvard.edu/press/2007/pr200722_images.html

THE MAGELLANIC CLOUDS ARE FIRST-TIME VISITORS

Cambridge, MA - The Large Magellanic Cloud (LMC) and Small Magellanic Cloud
(SMC) are two of the Milky Way's closest neighboring galaxies. A stunning sight
in the southern hemisphere, they were named after Ferdinand Magellan, who
explored those waters in the 16th century. For hundreds of years, these galaxies
were considered satellites of the Milky Way, gravitationally bound to our home
galaxy. New research by Gurtina Besla (Harvard-Smithsonian Center for
Astrophysics) and her colleagues shows that the Magellanic Clouds are recent
arrivals on their first visit to the Milky Way's neighborhood.

"We have known about the Clouds since the time of Magellan, and a single
measurement has thrown out everything we thought we understood about their
history and evolution," says Besla.

Although they look like glowing clouds to the unaided eye, the LMC and SMC are
both irregular galaxies. The Large Magellanic Cloud is located approximately
160,000 light-years from Earth. It's about one-twentieth as large as our galaxy
in diameter and holds about one-tenth as many stars. The Small Magellanic Cloud
is located around 200,000 light-years from Earth. It's about ten times smaller
than its companion and a hundred times smaller than the Milky Way.

Earlier this year, CfA astronomers reported measuring the 3-d velocities of the
Magellanic Clouds through space with greater accuracy than ever before. The
velocities were anomalously high. Two explanations were proposed: 1) the Milky
Way is more massive than previously thought, or 2) the Magellanic Clouds are not
gravitationally bound to the Milky Way. Further analysis by Besla and her
colleagues verified the second explanation. The parabolic orbit they calculated
for the Clouds, based on the observed velocities, shows that both are on their
first pass by the Milky Way.

This result carries several implications. For example, as a spiral galaxy the
Milky Way has a large gaseous disk intermixed with billions of stars. That
gaseous disk is known to be significantly warped, extending about 10,000
light-years above and below the galaxy's plane. Astronomers theorized that
gravitational tides due to previous passages of the Magellanic Clouds caused
this warp. However, since the Clouds arrived only 1-3 billion years ago, they
are not likely to be the source of the warp.

Another puzzle relates to the Magellanic Clouds themselves. A long trail of
hydrogen gas called the Magellanic Stream extends behind the Clouds, spanning
100 degrees of the sky from the earth's viewpoint. Some astronomers suggested
that the Magellanic Stream formed due to tidal interactions between the Clouds
and the Milky Way. Others believed that hydrogen was stripped from the Clouds by
gas pressure as they plunged through the extremely tenuous gas surrounding our
galaxy. A first-passage scenario rules out both scenarios.

"We've been left with a real mystery," says Besla. "One answer has led to many
more questions."

Finally, the star-forming history of the Clouds themselves must be revisited.
Rather than forming stars continuously like the Milky Way, the Magellanic Clouds
have undergone several bursts of star formation followed by long quiet periods.
Astronomers thought that the starbursts coincided with previous close passes by
the Milky Way. This explanation no longer holds true. Instead, interactions
between the SMC and LMC may be the primary force driving star formation in both
galaxies.

In the future, Besla and her colleagues intend to focus on the origin of the
Magellanic Stream, conducting N-body simulations to puzzle out possible
formation mechanisms. Other astronomers will make direct observations and survey
the Stream. The combined power of observational and theoretical research may
answer the questions generated by the current work.

The paper describing this work has been accepted for publication in the
Astrophysical Journal and is available online at

http://arxiv.org/abs/astro-ph/0703196.

Besla's co-authors were Nitya Kallivayalil, Lars Hernquist, T.J. Cox, and
Charles Alcock (Harvard-Smithsonian Center for Astrophysics); Brant Robertson
(Kavli Institute for Cosmological Physics, University of Chicago); and Roeland
P. van der Marel (STScI).

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.