Kennicutt-Schmidt relation of galaxies over 13 billion years in the COLIBRE hydrodynamical simulations

We investigate the correlation between star formation rate (SFR) surface density and gas surface density (known as the Kennicutt-Schmidt, KS, relation) at kiloparsec (kpc) scales across cosmic time (0 ≤ z ≤ 8) for galaxies with stellar masses >10⁹\, M, using the COLIBRE state-of-the-art cosmological hydrodynamical simulations. These simulations feature on-the-fly non-equilibrium chemistry coupled to dust grain evolution and detailed radiative cooling down to ≈10 K, enabling direct predictions for the atomic (H i) and molecular (H2) KS relations. At z ≈ 0, COLIBRE reproduces the observed (spatially-resolved) KS relations for H i and H2, including the associated scatter, which we predict to be significantly correlated with stellar surface density, local specific SFR (sSFR), and gas metallicity. We show that the H i KS relation steepens for lower-mass galaxies, while the H2 KS relation shifts to higher normalization in galaxies with higher sSFRs. The H2 depletion time decreases by a factor of ≈20 from z = 0 to z = 8, primarily due to the decreasing gas-phase metallicity. This results in less H2 and more H i being associated with a given SFR at higher redshift. We also find that galaxies with higher sSFRs have a larger molecular gas content and higher star formation efficiency per unit gas mass on kpc scales. The predicted evolution of the H2 depletion time and its correlation with a galaxy’s sSFR agree remarkably well with observations in a wide redshift range, 0 ≤ z ≤ 5.