The Formation of a Realistic Disk Galaxy in Λ‐dominated Cosmologies

We simulate the formation of a realistic disk galaxy within the hierarchicalscenario of structure formation and study its internal properties to thepresent epoch. We compare results from a LambdaCDM simulation with a LambdaWDM(2keV) simulation that forms significantly less small scale structure. We showhow high mass and force resolution in both the gas and dark matter componentsplay an important role in solving the angular momentum catastrophe claimed fromprevious simulations of galaxy formation within the hierarchical framework. Thestellar material in the disk component has a final specific angular momentumequal to 40% and 90% of that of the dark halo in the LambdaCDM and LambdaWDMmodels respectively. The LambdaWDM galaxy has a drastically reduced satellitepopulation and a negligible stellar spheroidal component. Encounters withsatellites play only a minor role in disturbing the disk. Satellites possess avariety of star formation histories linked to mergers and pericentric passagesalong their orbit around the primary galaxy. In both cosmologies, the galactichalo retains most of the baryons accreted and builds up a hot gas phase with asubstantial X-ray emission. Therefore, while we have been successful increating a realistic stellar disk in a massive galaxy within the LambdaCDMscenario, energy injection emerges as necessary ingredient to reduce the baryonfraction in galactic halos, independent of the cosmology adopted. (abridged)