Streams of stars switch galaxies

Hundreds of stars which were born in the Small Magellanic Cloud have been discovered hiding in neighbouring mini galaxy the Large Magellanic Cloud.

The LMC as observed by Spitzer, showing the stolen stars in red as they are moving away from us and blue as they are moving towards us
The LMC as observed by the Spitzer space telescope, showing the stolen stars in red as they are moving away from us and blue as they are moving towards us (STSI/AURA/NASA).

Astronomers at the National Optical Astronomy Observatory, along with their collaborators, obtained the spectra of thousands of giant stars in the Large Magellanic Cloud (LMC) using a spectrograph on the 4 metre Blanco telescope in Chile. Giant and supergiant stars were chosen for the survey as they are bright and thus easier to observe.

About 400 of these stars are suspected to have originated in the Small Magellanic Clouds (SMC). The LMC and SMC are both satellite galaxies to the Milky Way and can be seen with the naked eye from the Southern Hemisphere. The main reason for this suspicion is the unusual motions through space of these rogue stars. This motion, known as kinematics, relative to the LMC is not what is expected.

There are two possible scenarios to explain the strange observations. The first is that these stars are travelling through the LMC in a direction counter to that of the main population. The second is that the stars are moving in the same direction as the rest of the stars, but in a plane that is inclined at a large angle.

“We can’t say that the counter-rotating scenario is more likely than the highly inclined plane scenario”, explains Knut Olsen, lead author of an upcoming paper in the Astrophysical Journal. “We were only able to measure whether the stars are moving away from or towards us and at what speed, and we don’t know the physical geometry of the structure that the stars are in.”

In addition to the irregular motion, spectral analysis of the stars show that their chemical composition is akin to those in the SMC and does not match the rest of the stars in the LMC. This along with their distinct motion points to the fact that the LMC stole these stars from the SMC.

“The stars we determined have been stolen from the SMC have very similar kinematics to that of a streamer of hydrogen gas that connects the LMC with the SMC – we therefore think that the LMC has pulled both this gas and the stars we found from the SMC,” Knut told Skymania News.

Previously, it was though that this stream of hydrogen which connects the LMC and SMC was flowing out of the LMC. However, if the stars have indeed been plucked from the SMC then it means that this gas is actually flowing out of the SMC. This infall of material is most likely the source of an unexplained overdensity of hydrogen gas.

It also explains the presence of a vigorous star forming region, as shockwaves from the hydrogen streamer entering the LMC compress pockets of gas until it collapses into stars. The knowledge that the SMC stars moved in with their neighbours helps us to further our understanding of the Magellanic Clouds.

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By Amanda Doyle

I am an astrophysics postdoctoral research assistant at the University of Warwick. I obtained my PhD from Keele University in 2014 and my thesis title was "Spectral analyses of solar-like stars". My research involves refining stellar parameters with the aim of improving our understanding of both stars and planets. I completed my masters in astronomy at Swinburne University of Technology via the Swinburne Astronomy Online programme in 2010, and I obtained my degree in physics with astronomy from Dublin City University in 2008. When I'm not doing research, I like to write about all aspects of astronomy. I am a freelance science writer and I contribute to Astronomy Now, NASA's Astrobiology Magazine, BBC Sky at Night magazine, Skymania News, and Sen. I am also the editor of Popular Astronomy magazine.

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