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 th e 256 processors of the Silicon Graphics T3E massively parallel supercomput er hosted by the Cineca Super-computing Center (Bologna, Italy).(1) The code combines the smoothed particle hydrodynamics (SPH) method for solv ing hydrodynamical equations with the popular Barnes & Hut tree-code to per form 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 criteri on. Benchmarks, in terms of load balance and scalability, of the code are analy sed and critically discussed against the adiabatic collapse of an isotherma l gas sphere test using 2 x 10(4) particles on 8 processors. The code resul ts balance at more than the 95 per cent level. Increasing the number of pro cessors, the load balance slightly worsens. The deviation from perfect scal ability for increasing number of processors is almost negligible up to 32 p rocessors. Finally, we present a simulation of the formation of an X-ray ga laxy cluster in a flat cold dark matter cosmology, using 2 x 10(5) particle s and 32 processors, and compare our results with Evrard's (PM)-M-3-SPH sim ulations. Additionally we have incorporated radiative cooling, star formation, feedba ck from SNe of types II and Ia, stellar winds and UV flux from massive star s, and an algorithm to follow the chemical enrichment of the interstellar m edium. Simulations with some of these ingredients are also presented.