tokens: This work was supported by Sonderforschungsbereich SFB 881 ‘The Milky Way System’ (sub-projects B1, B2, B5 and B8) of the German Research Foundation (DFG) and SPP 1573, ‘Physics of the Interstellar Medium’ (grant number GL 668/2-1). In addition, RSK acknowledges support from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) via the ERC Advanced Grant STARLIGHT (project number 339177). The numerical simulations presented in this work were performed on the Milky Way supercomputer, which is funded by the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center (SFB 881) ‘The Milky Way System’ (subproject Z2) and hosted and co-funded by the Jülich Supercomputing Center (JSC). The authors thank the anonymous referee for helpful and constructive comments. CBM thanks Patrick Hennebelle, Pavel Kroupa, Tilman Hartwig and Scott Trager for helpful discussions.
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