Origine della natura duale dell'alone stellare della Milky Way

The theory of the Milky Way formation, in the framework of the Lambda CDM model, predicts galactic stellar halos to be built from multiple accretion events starting from the first structure to collapse in the Universe. Evidences in the past few decades have indicated that the Galactic halo consists in two overlapping structural components, an inner and an outer halo. We provide a set of numerical N-body simulation for studying the formation of the outer Milky Way stellar halo trough accretion events. After simulating minor mergers of a satellite halo with a Dark Matter main halo on prograde and retrograde orbits respect to the host disk, I analyse the signal left by satellite stars in the rotation velocity distribution. The aim is to explore the orbital conditions where a retrograde signal in the outer part of the halo can be obtained, in order to give a possible explanation of the observed rotational properties of the Milky Way stellar halo. My results show that the dynamical friction has a fundamental role in assembling the final velocity distributions originated by different orbits and that retrograde satellites moving on low inclination orbits deposit more stars in the outer halo regions and therefore can produce the counter-rotating behaviour observed in the outer Milky Way halo.