A&a Manuscript No. an Sph Code for Galaxy Formation Problems Presentation of the Code

We present and test a code for two-uid simulations of galaxy formation, one of the uids being collision-less. The hydrodynamical evolution is solved through the SPH method while gravitational forces are calculated using a tree method. The code is Lagrangian, and fully adaptive both in space and time. A signiicant fraction gas in simulations of hierarchical galaxy formation ends up in tight clumps where it is, in terms of computational eeort, very expensive to integrate the SPH equations. Furthermore, this is a computational waste since these tight gas clumps are typically not gravitationally resolved. We solve this by merging gas particles in these regions, thereby limiting the SPH resolution to the gravi-tational force resolution, and further speeding up the simulation through the reduction in the number of particles.