Momentum transfer across shear flows in smoothed particle hydrodynamic simulations of galaxy formation

We investigate the evolution of angular momentum in smoothed particle hydrodynamic (SPH) simulations of galaxy formation, paying particular attention to artificial numerical effects. We find that a cold gas disc forming in an ambient hot gas halo receives a strong hydrodynamic torque from the hot gas. By splitting the hydrodynamic force into artificial viscosity and pressure gradients, we find that the angular momentum transport is caused not by the artificial viscosity but by the pressure gradients. Using simple test simulations of shear flows, we conclude that the pressure gradient-based viscosity can be divided into two components: one due to the noisiness of SPH and the other due to ram pressure. The former is problematic even with very high resolution, because increasing the resolution does not reduce the noisiness. On the other hand, the ram pressure effect appears only when a cold gas disc or sheet does not contain enough particles. In such a case, holes form in the disc or sheet, and then ram pressure from intra-hole hot gas causes significant deceleration. In simulations of galactic disc formation, star formation usually decreases the number of cold gas particles, and hole formation leads to the fragmentation of the disc. This fragmentation not only induces further angular momentum transport, but also affects star formation in the disc. To circumvent these problems, we modify the SPH algorithm, decoupling the cold gas phases from the hot ones, i.e. inhibiting the hydrodynamic interaction between cold and hot particles. This, a crude modelling of a multiphase fluid in SPH cosmological simulations, leads to the formation of smooth extended cold gas discs and to better numerical convergence. The decoupling is applicable in so far as the self-gravitating gas disc with negligible external pressure is a good approximation for a cold gas disc.