October 29, 2012

Large Magellanic Cloud Caught Stealing Stars From Neighbor

Lee Rannals for - Your Universe Online

Astronomers using simulations were able to catch the Large Magellanic Cloud (LMC) in the act of stealing stars away from its neighbor, the Small Magellanic Cloud (SMC).

The team was trying to look for massive compact halo objects (MACHOs) in order to find out if they could be a major component of dark matter.

In order for MACHOs to make up dark matter, they must be so faint that they can't be directly detected.

When studying the LMC, the astronomers wanted to see MACHOs within the Milky Way lensing distant LMC stars.

"We originally set out to understand the evolution of the interacting LMC and SMC galaxies," lead author Gurtina Besla of Columbia University said. "We were surprised that, in addition, we could rule out the idea that dark matter is contained in MACHOs."

Avi Loeb of the Harvard-Smithsonian Center for Astrophysics said that instead of MACHOs, the team found a trail of stars removed from the SMC is responsible for the microlensing events first seen.

Microlensing is a phenomenon in which a nearby object passes in front of a more distant star. The gravity of the closer object bends light from the star like a lens, magnifying it and causing it to brighten.

However, while looking for this, they uncovered a thief in the night, snatching stars away from a neighboring galaxy.

Computer simulations showed that the most likely explanation for the observed microlensing events was an unseen population of stars removed by the LMC from its companion, the SMC.

"You could say we discovered a crime of galactic proportions," Loeb said.

Foreground stars in the LMC are gravitationally lensing the trail of removed stars located behind the LMC from the Earth's vantage point.

Only a fast-moving population of stars could prompt the observed rate and durations of the microlensing events, according to the astronomers.

The best way for astronomers to see this stellar population is in a galactic collision.

"By reconstructing the scene, we found that the LMC and SMC collided violently hundreds of millions of years ago. That's when the LMC stripped out the lensed stars," Loeb said.

Although they haven't found evidence for the trail of lensed stars, a number of teams are searching for the signatures of these stars within a bridge of gas that connects the galaxies.

The astronomers reported their findings from the simulations in the journal Monthly Notices of the Royal Astronomical Society.