News: Milky Way Companions Just Passing Through.
- From: Ye Old One <usenet@xxxxxxxxx>
- Date: Tue, 18 Sep 2007 23:01:23 GMT
Milky Way Companions Just Passing Through
http://news.yahoo.com/s/space/20070918/sc_space/milkywaycompanionsjustpassingthrough;_ylt=AiUPL7sp0gndwhcieKX_.FuFDvII
Jeanna Bryner
Staff Writer
SPACE.com Tue Sep 18, 7:15 AM ET
Two dwarf galaxies thought to be our Milky Way's longtime companions
are actually relative newcomers to our neighborhood that are just
passing through, according to a new study.
The surprising finding is a celestial curveball of sorts, sending
astronomers back to the clubhouse in order to rework theories that
were based on long-lasting interactions between the Milky Way and the
dwarf galaxies, called the Large and Small Magellanic Clouds.
"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," said the study's lead author,
Gurtina Besla of the Harvard-Smithsonian Center for Astrophysics in
Massachusetts.
For instance, some astronomers thought a blazing trail of hydrogen gas
extending from the Clouds, called the Magellanic Stream, formed due to
tidal interactions between the Clouds and the Milky Way. Others
explained the gas trail as the result of hydrogen being stripped from
the Clouds by gas pressure as they plunged through the gas halo around
our galaxy. Both scenarios are false if the galaxies are indeed just
passing through.
Glowing clouds
Located about 160,000 light-years from Earth, the Large Magellanic
Cloud (LMC) is only one-twentieth the diameter of our galaxy and
contains one-tenth as many stars. The Small Magellanic Cloud resides
200,000 light-years from Earth and is about 100 times smaller than the
Milky Way.
Earlier this year, astronomers making the most detailed measurements
yet of the 3-dimensional velocities of the Magellanic Clouds found
they are flying through space twice as fast as previously thought.
Besla's team incorporated the new estimates into computer models,
finding that both galaxies had extremely parabolic orbits and
indicated they had entered our neighborhood for the first time between
1 billion and 3 billion years ago.
"The problem is [the LMC] is moving at a velocity that would
correspond to a parabolic orbit," Besla explained. "It's just moving
too fast. If there were no other effects involved, it would just
slingshot away." She added that friction forces from the Milky Way's
gas halo and an observed loss of mass in the form of the Magellanic
Stream slow down the galaxies.
Even still, with such elongated orbits, the galaxies are unlikely to
boomerang back toward the Milky Way any time soon. "It will go out
really far before it comes back around again and it will take an
extremely long time ... on the order of like 8 billion years and
beyond," Besla told SPACE.com.
One answer, many questions
The results have implications for at least two astrophysical
phenomena.
Theories put forward to explain the Magellanic Stream involved a
lengthy interaction between the Clouds and our galaxy. An alternative
mechanism must be at work, Besla said.
The researchers suggest a type of stellar feedback. "As stars form
they start losing a lot of material through stellar winds and they
also explode and that blows out material," Besla said. "It's possible
some of that material gets puffed out and then other effects like 'ram
pressure' and tidal effects can then remove this really loosely bound
stuff."
Tidal effects between large objects (such as the moon and Earth, or
two galaxies) cause one side of an object to be tugged more than the
other side, stretching it.
In addition, the LMC and SMC have served as laboratories for
understanding how stars evolve. Unlike the Milky Way, which is
continually churning out stars, the Magellanic Clouds have undergone
several bursts of star formation followed by quiet periods.
"Those bursts had typically been linked to multiple passages around
the Milky Way," Besla said. "Now that doesn't fly."
--
Bob.
.
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