  We simulate the formation of a realistic disk galaxy within the hierarchical
scenario of structure formation and study its internal properties to the
present epoch. We compare results from a LambdaCDM simulation with a LambdaWDM
(2keV) simulation that forms significantly less small scale structure. We show
how high mass and force resolution in both the gas and dark matter components
play an important role in solving the angular momentum catastrophe claimed from
previous simulations of galaxy formation within the hierarchical framework. The
stellar material in the disk component has a final specific angular momentum
equal to 40% and 90% of that of the dark halo in the LambdaCDM and LambdaWDM
models respectively. The LambdaWDM galaxy has a drastically reduced satellite
population and a negligible stellar spheroidal component. Encounters with
satellites play only a minor role in disturbing the disk. Satellites possess a
variety of star formation histories linked to mergers and pericentric passages
along their orbit around the primary galaxy. In both cosmologies, the galactic
halo retains most of the baryons accreted and builds up a hot gas phase with a
substantial X-ray emission. Therefore, while we have been successful in
creating a realistic stellar disk in a massive galaxy within the LambdaCDM
scenario, energy injection emerges as necessary ingredient to reduce the baryon
fraction in galactic halos, independent of the cosmology adopted. (abridged)
