Star-formation efficiencies are assumed to be related to the thermal history of the gas effectively making the efficiencies large (∼ 1) at early times (z > 4–5) and much smaller (∼ 0.01) at later times. Early star-formation is assumed to be either a) self-propagating or b) only “local”, the former resulting in the strongest bursts and feedback events. The early, fast star formation is assumed to be triggered when the gas density of an SPH particle exceeds a certain critical value, chosen to be nH,fast = 0.3 cm−3. The conversion of such an SPH particle into a star particle may or may not trigger a burst of self-propagating star formation (SPSF) in the cold, dense gas surrounding it: in scenario a) not only the SPH particle which gets above the critical density threshold, but also its neighbouring cold and dense SPH particles with densities above nH,fast,low (< nH,fast) are triggered for conversion into star particles on their individual, dynamical timescales. Such SPSF is observed in some star-burst galaxies (e.g., in expanding super-shells — see Mori et al. 1997). In scenario b) only the initial SPH particle above the critical density threshold is triggered for star formation on the dynamical time scale.
We selected 12 dark matter halos from a cosmological ΛCDM N-body simulation for the galaxy formation simulations. The masses of these halos spanned more than a factor of 10 and their characteristic velocities V200 range from 100 to 250 km/s. After resampling the galaxy formation simulations consisted of 30000-150000 SPH+DM particles. We started out by running all 12 galaxy simulations using the SPSF prescription with a lower density threshold of nH,fast,low = 0.1 cm−3. Seven of the resulting galaxies at z = 0 had distinctly disk galaxy like morphologies and kinematics, the remaining 5 lenticular (S0) or elliptical like morphologies and kinematics. 4 additional series of simulations were subsequently run for the 7 disk galaxies: Three using again early SPSF with nH,fast,lower = 0.05, 0.2 and 0.25 cm−3 and one series with fast, early, but non-SPSF. The four choices of nH,fast,lower results in conversion of 2–5% of the gas in the simulations into stars in the early bursts. In the models without SPSF about 1% of the gas is turned into stars in the early bursts.