Unproductive Mergers

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Science  27 Nov 2009:
Vol. 326, Issue 5957, pp. 1164
DOI: 10.1126/science.326.5957.1164-a

How do you make a massive galaxy that does not form new stars anymore? According to van der Wel et al. the answer is by merging galaxies of similar size. They selected 17,480 galaxies with low star-formation rates from the millions of galaxies mapped in the Sloan Digital Sky Survey and analyzed their shape distribution. Whereas the projected axial ratios of the less massive galaxies were consistent with a mix of shapes, from bulge- to disk-dominated galaxies, the most massive galaxies in their sample all turned out to be spheroidal. Because the only known mechanism capable of forming round galaxies is the merging of galaxies with mass ratios up to about 3, the authors concluded that such merging is the dominant mechanism for the production of massive, non–star-forming galaxies.

Astrophys. J. 706, L120 (2009).

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