ner_tags: B-Person I-Person O O O O O O O O O O B-Organization I-Organization I-Organization I-Organization I-Organization B-Organization B-Organization I-Organization I-Organization I-Organization O O B-Organization I-Organization I-Organization O B-Organization O O O O O O B-Grant I-Grant I-Grant I-Grant O O B-Organization B-Organization I-Organization B-Person I-Person O O O O B-Event B-Event I-Event I-Event I-Event I-Event I-Event I-Event I-Event I-Event I-Event I-Event I-Event I-Event O O O O O O O O B-Person I-Person O B-Person I-Person O O B-Person I-Person O O O O O O O O B-Event O O O O O O O B-Person I-Person O O O O O O O O B-Person I-Person O B-Person I-Person O O O O O O O O O O O O O B-Person I-Person O B-Person I-Person O O O O O O O O O O O O O O O O O O O O B-Survey I-Survey I-Survey I-Survey I-Survey B-Survey O B-Survey O O O O O B-Organization I-Organization O B-Observatory I-Observatory I-Observatory I-Observatory I-Observatory O O O B-Organization I-Organization I-Organization O B-Grant I-Grant B-Grant O B-Grant O O O O O O O O O O O B-Telescope I-Telescope I-Telescope O O O O O B-Organization I-Organization I-Organization B-Organization I-Organization I-Organization I-Organization O O O O B-Organization O O O O O O B-Person I-Person O O O O O O O O O O O O B-Software O O O O O O O O O O O O O O O O O O
tokens: Guang-Xing Li received support for this research through a stipend from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne and is currently supported by the Priority Program 1573 ISM-SPP from the DFG (Deutsche Forschungsgemeinschaft). Guang-Xing Li would like to thank ISIMA (International Summer Institute for Modelling in Astrophysics, now Kavli Summer Program in Astrophysics) for facilitating collaborations, and would like to thank Nicolas Peretto and Patrick Hennebelle for discussions. Lee Hartman is acknowledged for giving a nice lecture in ISIMA, which motivates the thinking of gravity, and Di Li is acknowledged for providing references on angular momentum. Guang-Xing Li thanks Alexei Kritsuk for discussions and for his final push towards the publication of the results. Guang-Xing Li thanks Manuel Behrendt for discussions of gravitational instabilities and clumps in disk galaxies. This publication makes use of molecular line data from the Boston University-FCRAO Galactic Ring Survey (GRS). The GRS is a joint project of Boston University and Five College Radio Astronomy Observatory, funded by the National Science Foundation under grants AST-9800334, AST-0098562, and AST-0100793. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. We also would like to thank Thomas Robitaille for making his code publicly available. The research made use of the astropy package. The referee has to be acknowledged, whose comments have lead to very significant improvements in our paper.