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Originally published in Science Express on 19 January 2006
Science 24 February 2006:
Vol. 311. no. 5764, pp. 1129 - 1132
DOI: 10.1126/science.1122294
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Reports

Explaining the Color Distributions of Globular Cluster Systems in Elliptical Galaxies

Suk-Jin Yoon,1,2* Sukyoung Ken Yi,1,2 Young-Wook Lee1

The colors of globular clusters in most large elliptical galaxies are bimodal. This is generally taken as evidence for the presence of two cluster subpopulations that have different geneses. However, here we find that, because of the nonlinear nature of the metallicity-to-color transformation, a coeval group of old clusters with a unimodal metallicity spread can exhibit color bimodality. The models of cluster colors indicate that horizontal-branch stars are the main drivers behind the empirical nonlinearity. We show that the scenario gives simple and cohesive explanations for all the key observations and could simplify theories of elliptical galaxy formation.

1 Department of Astronomy and Center for Space Astrophysics, Yonsei University, Seoul 120-749, Korea.
2 Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK.

* To whom correspondence should be addressed. E-mail: sjyoon{at}galaxy.yonsei.ac.kr

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Science. ISSN 0036-8075 (print), 1095-9203 (online)