tokens: ACKNOWLEDGEMENTS We thank Volker Springel for helpful comments, and Dylan Nelson for providing videos of the merging histories of galaxies in Illustris to help us understand the processes better. LW acknowledges support from the National Natural Science Foundation of China grants program (No.11573031), and the National Key Program for Science and Technology Research and Development (2017YFB0203300). QG is supported by two National Natural Science Foundation of China grants (Nos 11573033 and 11622325).
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