A Spiral Unraveled

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Science  03 Aug 2012:
Vol. 337, Issue 6094, pp. 504
DOI: 10.1126/science.337.6094.504-b

The origin of the spiral arms seen in galaxies is not fully understood. One long-standing theory posits that these bright, long, thin regions are manifestations of a long-lived density wave that moves with a constant angular speed. In the inner part of the galaxy, the stars and gas move faster than this density wave; in the outer part, the reverse is true. As gas moves into regions of higher density, it can get compressed and form stars. According to the density wave theory, these newly formed stars should appear slightly ahead of the arm traced by gas in the inner part of the galaxy and slightly behind in the outer part. To test this prediction, Ferreras et al. analyzed the properties of 787 star-forming regions in the spiral structure of NGC 4321 (or M100), a nearby galaxy with well-defined spiral arms. Using observations at different wavelengths to trace recently formed stars of different ages, they found that the distribution of stellar ages around the spiral arms does not follow the radial trend predicted by theory. For this galaxy, the data are consistent instead with short-lived spiral arms that corotate with the gas and the stars—thus adding to the evidence against the spiral density wave theory.

Mon. Not. R. Astron. Soc. 10.1111/j.1365-2966.2012.21017.x (2012).

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