The satellites of the Milky Way – insights from semi-analytic modelling in a ΛCDM cosmology

Abstract

We combine the six high-resolution Aquarius dark matter simulations with a semi-analytic galaxy formation model to investigate the properties of the satellites of Milky Way-like galaxies. We find good correspondence with the observed luminosity function, luminosity–metallicity relation and radial distribution of the Milky Way satellites. The star formation histories of the dwarf galaxies in our model vary widely, in accordance with what is seen observationally. Some systems are dominated by old populations, whereas others are dominated by intermediate populations; star formation histories can either be continuous or more bursty. Ram-pressure stripping of hot gas from the satellites leaves a clear imprint of the environment on the characteristics of a dwarf galaxy. We find that the fraction of satellites dominated by old populations of stars matches observations well. However, the internal metallicity distributions of the model satellites appear to be narrower than observed. This may indicate limitations in our treatment of chemical enrichment, which is based on the instantaneous recycling approximation. We find a strong correlation between the number of satellites and the dark matter mass of the host halo. Our model works best if the dark matter halo of the Milky Way has a mass of ∼8 × 1011 M⊙, in agreement with the lower estimates from observations, but about a factor of 2 lower than estimates based on the Local Group timing argument or abundance matching techniques. The galaxy that resembles the Milky Way the most also has the best-matching satellite luminosity function, although it does not contain an object as bright as the Large or Small Magellanic Cloud. Compared to other semi-analytic models and abundance matching relations we find that central galaxies reside in less massive haloes, but the halo mass–stellar mass relation in our model is consistent both with hydrodynamical simulations and with recent observations.