Resolving shocks and filaments in galaxy formation simulations: effects on gas properties and star formation in the circumgalactic medium

Abstract

There is an emerging consensus that large amounts of gas do not shock heat in the circumgalactic medium (CGM) of massive galaxies, but instead pierce deep into haloes from the cosmic web via filaments. To better resolve this process numerically, we have developed a novel `shock refinement' scheme within the moving mesh code AREPO that adaptively improves resolution around shocks on-the-fly in galaxy formation simulations. We apply this to a massive $\sim10^{12}$ M$_\odot$ halo at $z=6$ using the successful FABLE model, increasing the mass resolution by a factor of 512. With better refinement there are significantly more dense, metal-poor and fast-moving filaments and clumps flowing into the halo, leading to a more multiphase CGM. We find a $\sim50$ per cent boost in cool-dense gas mass and a 25 per cent increase in inflowing mass flux. Better resolved accretion shocks cause turbulence to increase dramatically, leading to a doubling in the halo's non-thermal pressure support. Despite much higher thermalisation at shocks with higher resolution, increased cooling rates suppress the thermal energy of the halo. In contrast, the faster and denser filaments cause a significant jump in the bulk kinetic energy of cool-dense gas, while in the hot phase turbulent energy increases by up to $\sim150$ per cent. Moreover, HI covering fractions within the CGM increase by up to 60 per cent. Consequently star formation is spread more widely and we predict a population of metal-poor stars forming within primordial filaments that deep JWST observations may be able to probe.