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Adaptive Mesh Refinement Simulations of Galaxy Formation: Exploring Numerical and Physical Parameters

We carry out adaptive mesh refinement (AMR) cosmological simulations of Milky-Way mass halos in order to investigate the formation of disk-like galaxies in a Λ-dominated Cold Dark Matter model. We evolve a suite of five halos to z = 0 and find gaseous-disk formation in all; however, in agreement with previous SPH simulations (that did not include a subgrid feedback model), the rotation curves of all halos are centrally peaked due to a massive spheroidal component. Our standard model includes radiative cooling and star formation, but no feedback. We further investigate this angular momentum problem by systematically modifying various simulation parameters including: (i) spatial resolution, ranging from 1700 to 212 pc; (ii) an additional pressure component to ensure that the Jeans length is always resolved; (iii) low star formation efficiency, going down to 0.1%; (iv) fixed physical resolution as opposed to comoving resolution; (v) a supernova feedback model which injects thermal energy to the local cell; and (vi) a subgrid feedback model which suppresses cooling in the immediate vicinity of a star formation event. Of all of these, we find that only the last (cooling suppression) has any impact on the massive spheroidal component. In particular, a simulation with cooling suppression and feedback results in a rotation curve that, while still peaked, is considerably reduced from our standard runs.