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Abstract: We report the discovery of two extremely low luminosity globular clusters inthe Milky Way Halo. These objects were detected in the Sloan Digital Sky SurveyData Release 5 and confirmed with deeper imaging at the Calar Alto Observatory.The clusters, Koposov 1 and Koposov 2, are located at $\sim 40-50$ kpc andappear to have old stellar populations and luminosities of only $M V \sim -1$mag. Their observed sizes of $\sim 3$ pc are well within the expected tidallimit of $\sim$10 pc at that distance. Together with Palomar 1, AM 4 andWhiting 1, these new clusters are the lowest luminosity globulars orbiting theMilky Way, with Koposov 2 the most extreme. Koposov 1 appears to lie close todistant branch of the Sagittarius stream. The half-mass relaxation times ofKoposov 1 and 2 are only $\sim 70$ and $\sim 55$ Myr respectively 2 orders ofmagnitude shorter than the age of the stellar populations, so it would seemthat they have undergone drastic mass segregation. Since they do not appear tobe very concentrated, their evaporation timescales may be as low as $\sim 0.1t { m Hubble}$. These discoveries show that the structural parameter space ofglobular clusters in the Milky Way halo is not yet fully explored. They alsoadd, through their short remaining survival times, significant direct evidencefor a once much larger population of globular clusters.

Autor: S. Koposov 1,2, J. T. A. de Jong 1, V. Belokurov 2, H.-W. Rix 1, D. B. Zucker 2, N. W. Evans 2, G. Gilmore 2, M. J. Irwin 2, E. F


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