The Galaxy and its stellar halo: insights on their formation from a hybrid cosmological approach

We use a series of high-resolution simulations of a ‘Milky Way’ halo coupled to semi-analytic methods to study the formation of our own Galaxy and its stellar halo. The physical properties of our model Milky Way, as well as the age and metallicity distribution of stars in the different components, are in relatively good agreement with observational measurements. Assuming that the stellar halo builds up from the cores of the satellite galaxies that merged with the Milky Way over its lifetime, we are able to study the physical and structural properties of this component. In agreement with previous work, we find that the largest contribution to the stellar halo should come from a few relatively massive (108– 1010M⊙) satellites accreted at early times. Our ‘stellar halo’ does not exhibit any clear metallicity gradient, but higher metallicity stars are more centrally concentrated than stars of lower abundance. This indicates that the probability of observing low-metallicity halo stars increases with distance from the Galactic Centre. We find that the proposed ‘dual’ nature of the Galactic stellar halo can be explained in our model as a result of a mass–metallicity relation imprinted in the building blocks of this component.