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

ABSTRACT 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 ($0\le z \le 8$) for galaxies with stellar masses $\gt 10^9\, \rm M_{\odot }$, 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 $\approx 10$ K, enabling direct predictions for the atomic (H i) and molecular (H$_2$) KS relations. At $z\approx 0$, COLIBRE reproduces the observed (spatially resolved) KS relations for H i and H$_2$, 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 H$_2$ KS relation shifts to higher normalization in galaxies with higher sSFRs. The H$_2$ depletion time decreases by a factor of $\approx 20$ from $z = 0$ to $z = 8$, primarily due to the decreasing gas-phase metallicity. This results in less H$_2$ 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 H$_2$ depletion time and its correlation with a galaxy’s sSFR agree remarkably well with observations in a wide redshift range, $0\le z\le 5$.