Scaling relations of metallicity, stellar mass and star formation rate in metal-poor starbursts - II. Theoretical models

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 similar to 1100 galaxies from redshift 0 to greater than or similar to 3, spanning almost two orders of magnitude in metal abundance, a factor of similar to 10(6) in SFR and of similar to 10(5) 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-star. 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-star and SFR scaling relations up to z greater than or similar to 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.