Schmidt-Kennicutt relations in SPH simulations of disc galaxies with effective thermal feedback from SNe

We study several versions of the Schmidt-Kennicutt (SK) relation obtained for isolated spiral galaxies in TreeSPH simulations run with the GADGET3 code including the novel MUlti-Phase Particle Integrator (MUPPI) algorithm for star formation and stellar feedback. This is based on a sub-resolution multi-phase treatment of gas particles, where star formation is explicitly related to molecular gas, and the fraction of gas in the molecular phase is com-puted from hydrodynamical pressure, following a phenomenological correlation. No chemical evolution is included in this version of the code. The standard SK relation between surface densities of cold (neutral+molecular) gas and star formation rate of simulated galaxies shows a steepening at low gas surface densities, starting from a knee whose position depends on disc gas fraction: for more gas-rich discs the steepening takes place at higher surface densities. Be-cause gas fraction and metallicity are typically related, this environmental dependence mimics the predictions of models where the formation of H2 is modulated by metallicity. The cold gas surface density at which HI and molecular gas surface densities equate can range from ∼ 10 up to 34 M pc−2. As expected, the SK relation obtained using molecular gas show