A parallel TreeSPH code for galaxy formation

We describe a new implementation of a parallel TreeSPH code with the aim of simulating galaxy formation and evolution. The code has been parallelized using SHMEM, a Gray proprietary library to handle communications between the 256 processors of the Silicon Graphics T3E massively parallel supercomputer hosted by the Cineca Super-computing Center (Bologna, Italy).(1) The code combines the smoothed particle hydrodynamics (SPH) method for solving hydrodynamical equations with the popular Barnes & Hut tree-code to perform gravity calculation with an N x log N scaling, and it is based on the scalar TreeSPH code developed by Carraro et al. Parallelization is achieved by distributing particles along processors according to a workload criterion. Benchmarks, in terms of load balance and scalability, of the code are analysed and critically discussed against the adiabatic collapse of an isothermal gas sphere test using 2 x 10(4) particles on 8 processors. The code results balance at more than the 95 per cent level. Increasing the number of processors, the load balance slightly worsens. The deviation from perfect scalability for increasing number of processors is almost negligible up to 32 processors. Finally, we present a simulation of the formation of an X-ray galaxy cluster in a flat cold dark matter cosmology, using 2 x 10(5) particles and 32 processors, and compare our results with Evrard's (PM)-M-3-SPH simulations. Additionally we have incorporated radiative cooling, star formation, feedback from SNe of types II and Ia, stellar winds and UV flux from massive stars, and an algorithm to follow the chemical enrichment of the interstellar medium. Simulations with some of these ingredients are also presented.