Smooth particle hydrodynamics: importance of correction terms in adaptive resolution algorithms

We describe TREEASPH, a new code to evolve self-gravitating fluids, both with and without a collisionless component. In TREEASPH, gravitational forces are computed from a hierarchical tree algorithm (TREEcode), while hydrodynamic properties are computed by using a SPH method that includes the ∇h correction terms appearing when the spatial resolution h(t,r) is not a constant. Another important feature, which considerably increases the code efficiency on sequential and vectorial computers, is that time-stepping is performed from a PEC scheme (Predict–Evaluate–Correct) modified to allow for individual timesteps. Some authors have previously noted that the ∇h correction terms are needed to avoid the introduction on simulations of a non-physical entropy. By using TREEASPH we show here that, in cosmological simulations, this non-physical entropy has a negative sign. As a consequence, when the ∇h terms are neglected, the density peaks associated to shock fronts are overestimated. This in turn results in an overestimated efficiency of star-formation processes.