  We present and test a code for two-fluid simulations of galaxy formation, one
of the fluids 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
significant fraction gas in simulations of hierarchical galaxy formation ends
up in tight clumps where it is, in terms of computational effort, 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 gravitational force resolution, and further speeding up
the simulation through the reduction in the number of particles.
