tokens: −3 are the critical densities of the S(0):2–0 and S(1):3–1 rotational lines. r op is the ortho-H 2 /para-H 2 ratio (set to 3 which is the equilibrium value). For the cooling rates due to the [C ii ]158 μm and [O i ]63 μm+146 μm fine-structure lines we adopt the parametrizations by Röllig et al. ( 2006 ). The C ii cooling rate () is where a carbon to hydrogen abundance ratio that scales with metallicity according to χ [C] = 1.4 × 10 −4 Z ′ is assumed. For the parametrization of the O i cooling rate () we refer to equations (A.5) and (A.6) in Röllig et al. ( 2006 ) and simply note that we adopt (in accordance with Röllig et al. 2006 ) an oxygen to hydrogen abundance ratio of χ [O] = 3 × 10 −4 Z ′. Λ CO is the cooling rate due to CO rotational transitions. We use the parametrization provided by Papadopoulos Thi ( 2013 ): where χ CO /χ [C] is the relative CO to neutral carbon abundance ratio, the value of which we determine by interpolation, assuming that χ CO /χ [C] = (0.97, 0.98, 0.99, 1.0) for cm −3 , respectively (Papadopoulos Thi 2013 ). Λ gas–dust is the cooling rate due to gas–dust interactions and is given by (Papadopoulos et al. 2011 ) where the dust temperature ( T dust ) is calculated using equation ( 15 ) (Section 3.4.3 ). APPENDIX C: GMC MODELS Each SPH particle is divided into several GMCs as described in Section 3.4.1 , and we derive the molecular gas density and temperature within each from three basic parameters which are SFR density, GMC mass, m GMC , and metallicity, Z ′/ Z ⊙ . Derived from these basic parameters are the far-UV and CR field strengths, the H 2 gas mass fraction of each SPH particles, as well as the GMC properties used to derive the CO excitation and emission; H 2 density and temperature. Histograms of the basic parameters are shown in Fig. C1 , while properties derived thereof can be found in Fig. C2 . Figure C1. Mass-weighted histograms of the basic parameters of the GMCs in G1 (red solid), G2 (green dashed), and G3 (blue dotted). From top left and clockwise: the local far-UV field () (and CR ionization rate since ), GMC mass ( m GMC ), metallicity ( Z ′) and external pressure ( P ext ). Black vertical lines indicate the , m GMC , Z ′, P ext -values for which curves were calculated (see Fig. C2 ) – a total of 630 GMCs which make up our grid GMC models. Each GMC in the galaxies is assigned the curve of the GMC model at the closest grid point. Figure C2. Kinetic temperature versus H 2 density curves for 80 out of the 630 grid model GMCs that span the full (, M GMC , Z ′) parameter space set by and marked on top of the distributions in Fig. C1 (see also Section 3.4.4 ) for a pressure of K cm −3 . The grid model most often assigned to GMCs in G1 is indicated by the red dashed curve in the highlighted panel and corresponds to ( s −1 ), , and Z ′ = 3.2. In general, higher metallicity (from top to bottom) leads to more cooling via emission lines of ions, atoms and molecules, and hence lower temperatures. On the other
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