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

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 use a set of smoothed particle hydrodynamic (SPH) simulations, with a high dynamical range and force resolution, that include cooling, star formation, supernovae (SNe) feedback and a redshift-dependent UV background. We compare results from a Λ cold dark matter (ΛCDM) simulation to a Λ warm dark matter (ΛWDM) (2 keV) 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. Hence, a large disk forms without the need of strong energy injection, the z = 0 galaxies lie close to the I band Tully-Fisher relation, and the stellar material in the disk component has a final specific angular momentum equal to 40% and 90% of the dark halo in the ΛCDM and ΛWDM models, respectively. If rescaled to the Milky Way, the ΛCDM galaxy has an overabundance of satellites, with a total mass in the stellar halo 40% of that in the bulge+disk system. The ΛWDM 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 ΛCDM scenario, energy injection emerges as necessary ingredient to reduce the baryon fraction in galactic halos, independent of the cosmology adopted.