C. Purcell, J. Bullock, E. Tollerud, M. Rocha, and S. Chakrabarti. (2011)cite arxiv:1109.2918
Comment: Appears in Sept. 15 2011 issue of Nature. This version includes the
Letter published there, as well as the Supplementary Information hosted
online, for a total of 21 pages and 12 figures.