tokens: We thank the anonymous referee for a constructive review. We thank Simon Radford and William Tan for help with the CSO observation and data reduction, and Junhao Liu for helpful discussions on RADEX modeling. X.L. acknowledges the support of a Smithsonian Predoctoral Fellowship and the program A for outstanding PhD candidate of Nanjing University. J.M.D.K. gratefully acknowledges support in the form of an Emmy Noether Research Group from the Deutsche Forschungsgemeinschaft (DFG), grant number KR4801/1-1. C.B. is supported by the National Science Foundation under Award No. 1602583. Z.Y.Z. acknowledges support from the European Research Council in the form of the Advanced Investigator Programme, 321302, cosmicism . We acknowledge the support by the National Natural Science Foundation of China (grant No. 11629302). This research made use of Astropy, a community-developed core Python package for Astronomy (Astropy Collaboration et al. 2013 ), and NASA’s Astrophysics Data System. This material is based upon work at the Caltech Submillimeter Observatory, which was operated by the California Institute of Technology. This publication is also based on data acquired with the Atacama Pathfinder Experiment (APEX). APEX is a collaboration between the Max-Planck-Institut fur Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory. Facilities: SMA , VLA , CSO , APEX , GBT . Software: CASA, MIRIAD, MIR, GILDAS/CLASS, Astropy (Astropy Collaboration et al. 2013 ), RADEX (van der Tak et al. 2007 ), myRadex, correl_images.pro.
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