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Stellar thief’s
secret stash uncovered

Posted: 20 July 2011

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The Large Magellanic Cloud (LMC) has been caught red-handed as the perpetrator of a stellar crime involving the theft of hundreds of stars from the Small Magellanic Cloud (SMC), by astronomical detectives from the National Optical Astronomy Observatory (NOAO) and their collaborators at the University of Arizona and the Space Telescope Science Institute.

The LMC and SMC are both neighbouring galaxies to our Milky Way and lie at distances of around 160,000 and 200,000 light years away where they are easily visible to the unaided eye from the southern hemisphere.

Our Milky Way’s neighbouring galaxy, the Large Magellanic Cloud (LMC), has accreted many stars from its smaller neighbour, the Small Magellanic Cloud (SMC). The image shows the LMC as it appears in observations by the Spitzer Space Telescope. The overlaid red and blue dots represent velocities of stars moving away and towards us and whose origin has been traced to the SMC. Image Credit: Karl Gordon and Margaret Mexiner (Space Telescope Science Institute/AURA/NASA). Compilation by K. Olsen (NOAO/AURA/NSF).

With the analysis of approximately 5900 giant and supergiant stars in the LMC galaxy, NOAO astronomers Knut Olsen and Bob Blum alongside their collaborators Dennis Zaritsky (University of Arizona) and Martha Boyer and Karl Gordon (Space Telescope Science Institute) uncovered that over 5% of observed stars residing in the LMC are rotating counter to the rest of the stars that make up the galaxy, or perhaps in a plane that is greatly inclined to the rotation of the LMC. The stars’ unusual orbits indicate that they were not formed from the rotation and eventual collapse of a cloud of gas that is believed to have formed the LMC, an irregular galaxy which houses the Tarantula Nebula, the most active star-forming region in our Local Group.

Despite their findings, however, an ambiguity still remains; the astronomers were only able to measure the projection of stellar velocities into the line of sight, and no other directions in which the stars choose to move. “We think that the stolen stars have such peculiar velocities because they were stripped from the SMC in its interaction with the LMC,” says Olsen. “Gurtina Besla, a graduate student at Harvard, has done a computer simulation of the interaction and has demonstrated that the LMC and SMC passed very close to each other about 1.2 billion years ago and, in the process, pulled out a long stream of hydrogen gas from the SMC [the Magellanic Stream]. We suspect that the same event pulled out these stars.”

Satellite galaxies, the LMC and SMC, glow brightly on the left in the Chilean Atacama Desert - the site for ESO’s Very Large Telescope. The telescope stands to the right of the images while the Milky Way takes the centre. Image Credit: ESO/Y. Beletsky

Under closer examination, the counter-rotating stars admitted another anomaly. The chemical composition of these stars were different in comparison to the make-up of the other stars which rest within the large cloud. “Their chemical abundances don’t match those usually found in the LMC,” adds Olsen. “These stars are poorer in iron; but they do match the chemical abundances of the SMC.”

Armed with these pieces of evidence, the research team concluded that the galaxy was guilty of stealing stars from the SMC by its superior gravitational pull. Using the multi-object spectrometer on the Cerro Tololo Inter-American Observatory 4-metre Bianco Telescope in Chile to observe 4600 stars, the astronomers were able to observe their spectral characteristics simultaneously, finally combing these observations with data on 1300 other stars in their search for the uncovered pattern. “It is not always easy to tell whether the stars in a galaxy formed in the galaxy or formed somewhere else and then were captured,” says Olsen. “Since the LMC is so close to us, we were able to observe a large number of individual stars and to our surprise, the LMC contained a significant number of stars that must have formed elsewhere.”

The Spitzer Space Telescope also proved useful for making infrared observations in the team’s investigation of how stars form and evolve in the LMC – a very important angle from which the scientists had to approach. “Using observations with the Spitzer Space Telescope, we are able to get a complete census of the stellar populations in the LMC,” says Deputy Director Bob Blum. “With the ground-based observations we could determine the properties and motions of a large sample of stars throughout the galaxy. By combining both, we were able to tell that some of the stars must have come from the neighbouring SMC. This led to a deeper understanding of how galaxies can and do interact and change over time.”

This result could also help to explain the unusually large amount of star formation in the LMC in the Tarantula Nebula, named because of its appearance in a small telescope. If this region of star birth was in our Galaxy, lying as close as the Orion Nebula does (about 1,340 light years), then its glow would cast shadows on the ground due to its size. The stellar nursery is located where gas and captured stars torn from the SMC are colliding with the LMC’s own gas at high velocity. “We estimate the velocity with which the SMC gas hits the LMC to be about 50 to 100 km/s,” says Olsen. “We can’t know for sure, since we only observe the component of the velocity vector that is projected into the line of sight.” The shock wave that results from the collision pressurises the gas such that star formation is much more likely leading to bursts of large, unstable stars which can explode as supernova, where their older remnants can be identified with the help of X-ray telescopes.