HOW UNIVERSAL IS THE ΣSFR-ΣH2 RELATION?

It is a well-established empirical fact that the surface density of the star formation rate, Sigma(SFR), strongly correlates with the surface density of molecular hydrogen, Sigma(H2), at least when averaged over large (similar to kpc) scales. Much less is known, however, about whether (and how) the Sigma(SFR)-Sigma(H2) relation depends on environmental parameters, such as the metallicity or the UV radiation field in the interstellar medium (ISM). Furthermore, observations indicate that the scatter in the Sigma(SFR)-Sigma(H2) relation increases rapidly with decreasing averaging scale. How the scale-dependent scatter is generated and how one recovers a tight similar to kpc scale Sigma(SFR)-Sigma(H2) relation in the first place is still largely debated. Here, these questions are explored with hydrodynamical simulations that follow the formation and destruction of H-2, include radiative transfer of UV radiation, and resolve the ISM on similar to 60 pc scales. We find that within the considered range of H-2 surface densities (10-100 M-circle dot pc(-2)), the Sigma(SFR)-Sigma(H2) relation is steeper in environments of low-metallicity and/or high-radiation fields (compared to the Galaxy), that the star formation rate (SFR) at a given H-2 surface density is larger, and the scatter is increased. Deviations from a universal Sigma(SFR)-Sigma(H2) relation should be particularly relevant for high-redshift galaxies or for low-metallicity dwarfs at z similar to 0. We also find that the use of time-averaged SFRs produces a large, scale-dependent scatter in the Sigma(SFR)-Sigma(H2) relation. Given the plethora of observational data expected from upcoming surveys such as ALMA, the scale-scatter relation may indeed become a valuable tool for determining the physical mechanisms connecting star formation and H-2 formation.