Hot gas accretion fuels star formation faster than cold accretion in high-redshift galaxies

ABSTRACT We use high-resolution ($\simeq$35pc) hydrodynamical simulations of galaxy formation to investigate the relation between gas accretion and star formation in galaxies hosted by dark matter haloes of mass $10^{12}$${{\mathrm{M}}_{\odot }}$ at $z = 2$. At high-redshift, cold-accreted gas is expected to be readily available for star formation, while gas accreted in a hot mode is expected to require a longer time to cool down before being able to form stars. Contrary to these expectations, we find that the majority of cold-accreted gas takes several hundred Myr longer to form stars than hot-accreted gas after it reaches the inner circumgalactic medium (CGM). Approximately 10 per cent of the cold-accreted gas flows rapidly through the inner CGM on to the galactic disc. The remaining 90 per cent is trapped in a turbulent accretion region that extends up to $\sim 50$ per cent of the virial radius, from which it takes several hundred Myr for the gas to be transported to the star-forming disc. In contrast, most hot shock-heated gas avoids this ‘slow track’, and accretes directly from the CGM on to the disc where stars can form. We find that shock-heating of cold gas after accretion in the inner CGM and supernova-driven outflows contribute to, but do not fully explain, the delay in star formation. These processes combined slow down the delivery of cold-accreted gas to the galactic disc and consequently limit the rate of star formation in Milky Way mass galaxies at $z \gt 2$.