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."
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