2006MNRAS.365.1309H

We search for signatures of past accretion events in the Milky Way in the recently published catalogue by Nordström et al., containing accurate spatial and kinematic information as well as metallicities for 13240 nearby stars. To optimize our strategy, we use numerical simulations and characterize the properties of the debris from disrupted satellites. We find that stars with a common progenitor should show distinct correlations between their orbital parameters; in particular, between the apocentre (A) and pericentre (P), as well as their z-angular momentum (L_z). In the APL space, such stars are expected to cluster around regions of roughly constant eccentricity.

The APL space for the Nordström catalogue exhibits a wealth of substructure, much of which can be linked to dynamical perturbations induced by spiral arms and the Galactic bar. However, our analysis also reveals a statistically significant excess of stars on orbits of common (moderate) eccentricity, analogous to the pattern expected for merger debris. Besides being dynamically peculiar, the 274 stars in these substructures have very distinct metallicity and age distributions, providing further evidence of their extragalactic provenance. It is possible to identify three coherent groups among these stars, that, in all likelihood, correspond to the remains of disrupted satellites. The most metal-rich group ([Fe/H] > -0.45dex) has 120 stars distributed into two stellar populations of ∼8Gyr (33 per cent) and ∼12Gyr (67 per cent) of age. The second group with <[Fe/H]>∼-0.6dex has 86 stars and shows evidence of three populations of 8Gyr (15 per cent), 12Gyr (36 per cent) and 16Gyr (49 per cent) of age. Finally, the third group has 68 stars, with typical metallicity around -0.8dex and a single age of ∼14Gyr. The identification of substantial amounts of debris in the Galactic disc whose origin can be traced back to more than one satellite galaxy, provides evidence of the hierarchical formation of the Milky Way.