tokens: We thank Sera Markoff, Heino Falcke, and Geoffrey Bower for constructive discussions and the anonymous referee for useful comments. This work was supported in part by the Natural Science Foundation of China (grants 11103061, 11133005, 11121062, and 11103059), the National Basic Research Program of China (973 Program, grant 2014CB845800), and the Strategic Priority Research Program “The Emergence of Cosmological Structures” of the Chinese Academy of Sciences (grant XDB09000000).
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