2012MNRAS.427.1075M

Scaling relations of metallicity (O/H), star formation rate (SFR) and stellar mass (M_star) give important insight on galaxy evolution. They are obeyed by most galaxies in the Local Universe and also at high redshift. In a companion paper, we compiled a sample of ∼ 1100 galaxies from redshift 0 to ≳ 3, spanning almost two orders of magnitude in metal abundance, a factor of ∼ 10⁶ in SFR and of ∼ 10⁵ in stellar mass. We have characterized empirically the star formation `main sequence' (SFMS) and the mass–metallicity relation (MZR) for this sample, and also identified a class of low-metallicity starbursts, rare locally but more common in the distant Universe. These galaxies deviate significantly from the main scaling relations, with high SFR and low metal content for a given M<sub>star</sub>. In this paper, we model the scaling relations and explain these deviations from them with a set of multi-phase chemical evolution models based on the idea that, independently of redshift, initial physical conditions in a galaxy's evolutionary history can dictate its location in the scaling relations. Our models are able to successfully reproduce the O/H, M<sub>star</sub> and SFR scaling relations up to z ≳ 3, and also successfully predict the molecular cloud fraction as a function of stellar mass. These results suggest that the scaling relations are defined by different modes of star formation: an `active' starburst mode, more common at high redshift, and a quiescent `passive' mode that is predominant locally and governs the main trends.