Conservation laws in smooth particle hydrodynamics:: The DEVA code

We describe DEVA, a multistep AP3M-like SPH code particularly designed to study galaxy formation and evolution in connection with the global cosmological model. This code uses a formulation of SPH equations that ensures both energy and entropy conservation by including the so-called delh terms. Particular attention has also been paid to angular momentum conservation and to the accuracy of our code. We find that in order to avoid unphysical solutions, our code requires that cooling processes be implemented in a non-multistep way. We detail various cosmological simulations that have been performed to test DEVA and also to study the influence of the delh terms. Our results indicate that such correction terms have a non-negligible effect on some cosmological simulations, especially on high-density regions associated with either shock fronts or central cores of collapsed objects. Moreover, they suggest that codes paying particular attention to the implementation of conservation laws of physics at the scales of interest can attain good accuracy levels in conservation laws with limited computational resources. Another important result of this work is that the combined effects of entropy violation and multistep cooling implementation in cosmological simulations can be particularly dramatic concerning the mass distribution in the objects they produce.