Brief Encounter

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Science  26 Oct 2007:
Vol. 318, Issue 5850, pp. 535
DOI: 10.1126/science.318.5850.535a

The Small and Large Magellanic Clouds are our nearest galactic neighbors, visible in southern skies as thumb-sized smudges on the sky. Recent tracking with the Hubble Space Telescope showed that they are circling the Milky Way faster than was once thought, closely approaching the Milky Way's escape velocity. Besla et al. model the past motions of the Magellanic Clouds using the updated speeds and latest cosmological parameters and find that they are probably interlopers on their first pass of the Milky Way, rather than long-term companions on continuous orbits. This brief encounter scenario resolves some questions but raises others. It may explain why there has been an upsurge in star formation in the Clouds within the past few billion years. However, it also means that the Clouds may have had less of an effect on the diffuse hydrogen envelope of the Milky Way than had been assumed, as they have not been around long enough to warp the edges of the pancake of hydrogen in which the Milky Way sits or to pull out the Magellanic Stream, a band of hydrogen gas that almost circles the Milky Way and seems to follow the Clouds. — JB

Astrophys. J. 668, 949 (2007).

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