Whence the Spirals?

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Science  23 Apr 2010:
Vol. 328, Issue 5977, pp. 406
DOI: 10.1126/science.328.5977.406-c

Observations of the Milky Way and its nearest spiral neighbor, Andromeda, suggest that the outer stellar regions of these galaxies assembled by accretion of smaller, satellite galaxies. How common is this process in shaping such large spiral galaxies? Using the Subaru Telescope in Hawaii, Mouhcine et al. surveyed the outer regions of NGC 891, a spiral galaxy analogous to the Milky Way but located 30 million light-years away and thus well outside our immediate vicinity. The map of stars around this galaxy shows signs of tidal interactions and satellite disruption, including arc-like streams, one of which loops all the way around the galaxy. Together with observational evidence from other galaxies, this result suggests that accretion of small satellites may indeed be a common process in the formation of spiral galaxies. NGC 891 is also surrounded by a thick envelope of stars, which may have resulted from tidal disruption of several small satellite galaxies. This may also be a common property of large spiral galaxies.

Astrophys. J. 714, L12 (2010).

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