November 23, 2015 report
Scientists detect stellar streams around Magellanic Clouds
"Even though a prominent gaseous stream emanating from the clouds has been known and studied for some time, no obvious stellar streams had been found until recently," Vasily Belokurov, one of the co-authors of the paper, told Phys.org.
Belokurov, together with his colleague Sergey Koposov, used the Dark Energy Survey (DES) to track down stellar debris on the outskirts of the Magellanic Clouds. They were searching for the Magellanic stellar halo substructure using blue horizontal-branch (BHB) stars as tracers. BHBs are old and metal-poor stars powered by helium fusion that appear blue. They were chosen by Cambridge scientists as these stars suffer little contamination from other stellar populations. BHBs can be easily picked up and are one of the best stellar standard rulers available.
"Thanks to their unique properties, BHBs have proven to be a powerful tool to scrutinize the galactic halo out from the core to its far-flung fringes," the researchers wrote in the paper.
"In the halo, not only can these old and metal-poor stars be easily identified above the foreground of other populations thanks to their peculiar color, they are also one of the best stellar distance estimators available," they added.
DES is an astronomical survey specifically designed to measure the expansion history of the universe. It has one of the widest fields of view available for ground-based optical and infrared imaging. The scientists used DES' Year 1 public dataset for this study.
"To study the stellar halo substructure around the Magellanic Clouds, we use the photometric catalogs obtained from the publicly released DES Year 1 imaging, in particular, the latest improved version of the reduction," the paper reads.
Scanning many BHBs, the astronomers detected the stellar halo of the Magellanic system and its substructures. Each of these substructures is different in shape, extent and luminosity, and deserves its own detailed analysis.
The discovery of these stellar halo substructures led the scientists to ponder on the possible revision of our current knowledge about LMC's mass. They ask whether the LMC could, in fact, be much more massive than has been previously assumed.
"Our discoveries imply that the Large Magellanic Cloud might have been a lot more massive than we previously thought. To figure out exactly how much more massive, we need to follow these streams up with spectroscopy in order to measure their velocities," Belokurov said.
The researchers also noted that a combination of the deep imaging and spectroscopic follow-up of the tidal debris could provide more information about the orbital history of the Magellanic Clouds. This could be crucial to our understanding of the Clouds' future as they are in the process of merging with our Milky Way galaxy. They are currently on their way to join the already crowded Milky Way halo.
Using Blue Horizontal Branch stars identified in the Dark Energy Survey Year 1 data, we report the detection of an extended and lumpy stellar debris distribution around the Magellanic Clouds. At the heliocentric distance of the Clouds, overdensities of BHBs are seen to reach at least to ~30 degrees, and perhaps as far as ~50 degrees from the LMC. In 3D, the stellar halo is traceable to between 25 and 50 kpc from the LMC. We catalogue the most significant of the stellar sub-structures revealed, and announce the discovery of a number of narrow streams and diffuse debris clouds. Two narrow streams appear approximately aligned with the Magellanic Clouds' proper motion. Moreover, one of these overlaps with the gaseous Magellanic Stream on the sky. Curiously, two diffuse BHB agglomerations seem coincident with several of the recently discovered DES satellites. Given the enormous size and the conspicuous lumpiness of the LMC's stellar halo, we speculate that the dwarf could easily have been more massive than previously had been assumed.
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