Parallel Tree-SPH: A tool for galaxy formation

We describe a new implementation of a parallel Tree-SPH code with the aim o f simulating galaxy formation and evolution. The code has been parallelized using SHMEM, a Cray proprietary library to handle communications between t he 256 processors of the Silicon Graphics T3E massively parallel supercompu ter hosted by the Cineca Super-computing Center (Bologna, Italy). The code combines the smoothed particle hydrodynamics (SPH) method to solve hydrodyn amical equations with the popular Barnes and Hut (1986) tree-code to perfor m gravity calculation with a N x log N scaling, and it is based on the scal ar Tree-SPH code developed by Carraro et al. (1998). Parallelization is ach ieved by distributing particles along processors according to a workload cr iterion. Benchmarks of the code, in terms of load balance and scalability, are analysed and critically discussed against the adiabatic collapse of an isothermal gas sphere test using 2 x 10(4) particles on eight processors. T he code turns out to be balanced at more than 95% level. If the number of p rocessors is increased, the load balance worsens slightly. The deviation fr om perfect scalability at increasing number of processors is negligible up to 64 processors. Additionally we have incorporated radiative cooling, star formation, feedback and an algorithm to follow the chemical enrichment of the interstellar medium.