tokens: to M 0 , the total mass at t 0. The drop of the total mass over time is due to accretion of gas by sink particles. The vertical dashed line marks the onset of star formation. Interestingly, the evolution of the atomic hydrogen mass fraction is almost identical in runs FSN and FWSN, with a value around 40percent. Simulation FRWSN, however, has a mass fraction of atomic hydrogen around 70percent. This is due to the effect of photodissociation, which converts molecular into atomic gas. The mass fraction of ionized hydrogen starts to grow monotonically 10Myr after the onset of star formation from 1percent to 7percent in run FSN. This is the result of the supernova explosions, which produce an increasing amount of ionized gas. Run FWSN has a roughly constant ionized hydrogen mass fraction of 2percent. Due to the reduced SFR in this simulation, many fewer supernovae explode compared to run FSN, and the additional collisional ionization by stellar wind feedback cannot compensate for this effect. The ionized hydrogen mass fraction of run FRWSN oscillates between 3percent and 10percent as a function of time. These oscillations are very closely correlated with the variability in the radiative output from the star clusters (compare Fig. 11 ). This clearly demonstrates that photoionization from stellar radiation is the primary source of ionized gas in this simulation, with supernovae and winds contributing only a negligible amount. The molecular hydrogen mass fraction of run FSN drops from 60percent to 40percent from 40Myr onwards. The delay of 10Myr between the onset of star formation and the reduction of the molecular hydrogen mass fraction suggests that it is due to supernova feedback, since the evolution of the atomic and ionized hydrogen mass fractions show a similar behaviour. In contrast, if this reduction was primarily due to accretion of molecular gas on to the sink particles, one would not expect such a delay. For run FWSN, the molecular hydrogen mass fraction stays at around 60percent. In run FRWSN, the molecular hydrogen mass fraction oscillates around a value of 20percent. These oscillations are again indicative of photoevaporation processes caused by the stellar irradiation of the molecular clouds. 8 DEPLETION TIME Since we dynamically form H 2 in the simulations, it is interesting to check whether the relation between the molecular gas surface density H 2 and the SFR surface density SFR obtained in the simulations is in agreement with observations. The ratio of these two quantities is the depletion time . Using SFR from Fig. 7 and the instantaneous H 2 measured in face-on projections of the disc, we can plot t dep as a function of time for the three simulations. The result is shown in Fig. 14 . Figure 14. Top: depletion time for the three simulations as a function of time t . Middle: plot of SFR versus H 2 for the three simulations for all snaphots after t 40Myr. The dashed lines show constant depletion times of 0.1, 1 and 10Gyr (from
ner_tags: O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O B-Model O O O O O B-Model I-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Model O O O O O O O O O O O B-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Formula I-Formula O O O O O O O O O O O O O