Numerical modeling of galaxy evolution

A very significant problem in the modeling of disk-galaxy formation in the cold dark matter (CDM) cosmology is the so-called `angular momentum problem'. This problem arises when we numerically model the collapse of baryons within a dark halo in the CDM model. The formed baryonic disk has much less angular momentum than observed disk galaxies due to the considerable loss of angular momentum during the progressive merger of small clumps. As a result of efficient radiative cooling, the gas component collapses too deeply within the dark halo. When two such systems are merging, the angular momentum of the material near the center is effectively transported outwards by the tidal force. This is a physical reason for this problem, however, there may be a numerical origin due to the nature of the Smoothed Particle Hydrodynamics (SPH) method widely used in galaxy formation models. To address the numerical origin of the `angular momentum problem' with a much higher-resolution SPH model, we are developing our Parallel Tree-SPH code. After evolving four initial models with different mass and force resolution, we compare the angular momentum content of SPH particles. We find that both mass and force resolutions clearly affect the evolution of radiative cosmological SPH models. In most previous radiative cosmological SPH models, a mass ratio between SPH and dark matter particles is .However, we find that this mass ratio is a crucial parameter when we consider the angular momentum content of SPH particles and it is better to make the mass ratio ∼ 1.0 in such models. This revised version was published online in August 2006 with corrections to the Cover Date.