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tokens: We wish to thank Sourav Chatterjee for carefully reading the manuscript and providing useful comments that helped to improve this paper. We wish to thank Rainer Spurzem and Sverre Aarseth for their advice regarding the adjustment of the nbody6 software package for our study. LW and XCZ were supported by the Department of Astronomy at Peking University and the Kavli Institute for Astronomy and Astrophysics. MBNK was supported by the Peter and Patricia Gruber Foundation through the PPGF fellowship, by the Peking University One Hundred Talent Fund (985), and by the National Natural Science Foundation of China (grants 11010237, 11050110414, 11173004). This publication was made possible through the support of a grant from the John Templeton Foundation and National Astronomical Observatories of Chinese Academy of Sciences. The opinions expressed in this publication are those of the author(s) do not necessarily reflect the views of the John Templeton Foundation or National Astronomical Observatories of Chinese Academy of Sciences. The funds from John Templeton Foundation were awarded in a grant to The University of Chicago which also managed the programme in conjunction with National Astronomical Observatories, Chinese Academy of Sciences. RPC was supported by the Swedish Research Council (grants 2012-2254 and 2012-5807). MBD was supported by the Swedish Research Council (grants 2008-4089 and 2011-3991).