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 cosmological hydrodynamical simulations, which share the same initial conditions, in a hierarchical clustering scenario. In the first simulation, a simple and generic implementation of star formation has allowed galaxy-like objects with stellar bulges and extended, populated disks to form. Gas in the disk comes both from particles that survive mergers, keeping in part their angular momentum content, and from new gas supplied by infall once the merger process is over, with global specific angular momentum conservation. The stellar bulge forms from gas that has lost most of its angular momentum. In the second simulation, no star formation has been included. In this case, objects consist of an overpopulated central gas concentration and an extended, underpopulated disk. The central concentration forms from particles that suffer an important angular momentum loss in violent events, and it often contains 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. The difference between these two simulations is likely to be due to the stabilizing character of the stellar bulge-like cores that form in the first simulation, which diminishes the inflow of gas triggered by mergers and interactions.