Disk formation in hierarchical hydrodynamical simulations: A way out of the angular momentum catastrophe

We report results on the formation of disklike structures in two cosmologic al hydrodynamical simulations, which share the same initial conditions, in a hierarchical clustering scenario. In the first simulation, a simple and g eneric implementation of star formation has allowed galaxy-like objects wit h stellar bulges and extended, populated disks to form. Gas in the disk com es both from particles that survive mergers, keeping in part their angular momentum content, and from new gas supplied by infall once the merger proce ss is over, with global specific angular momentum conservation. The stellar bulge forms from gas that has lost most of its angular momentum. In the se cond simulation, no star formation has been included. In this case, objects consist of an overpopulated central gas concentration and an extended, und erpopulated disk. The central concentration forms from particles that suffe r an important angular momentum loss in violent events, and it often contai ns more than 70% of the object's baryonic mass. The external disk forms by late infall of gas that roughly conserves its specific angular momentum. Th e difference between these two simulations is likely to be due to the stabi lizing character of the stellar bulge-like cores that form in the first sim ulation, which diminishes the inflow of gas triggered by mergers and intera ctions.