Space.com : Milky Way Companions Just Passing Through

Milky Way Companions Just Passing Through By Jeanna Bryner Staff Writer posted: 18 September 2007 06:52 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."