SFR vs. gas surface density. Model results are displayed for three different epochs, after 4 Gyr (green curve), 8 Gyr (blue) and 12 Gyr (red). The last is compared to the observationally inferred SFR profile of the Milky Way (dotted magenta curve), obtained as discussed in Appendix B. The data points concern extragalactic measurements, compiled by Krumholz et al. (2012) (see also Fig. B.2). The dotted line is .