On the identification of merger debris in the Gaia era

We model the formation of the Galactic stellar halo via the accretion of satellite galaxies on to a time-dependent semicosmological galactic potential. Our goal is to characterize the substructure left by these accretion events in a close manner to what may be possible with the Gaia mission. We have created a synthetic Gaia solar neighbourhood catalogue by convolving the six-dimensional phase-space coordinates of stellar particles from our disrupted satellites with the latest estimates of the Gaia measurement errors, and included realistic background contamination due to the Galactic disc(s) and bulge. We find that, even after accounting for the expected observational errors, the resulting phase space is full of substructure. We are able to successfully isolate roughly 50 per cent of the different satellites contributing to the ‘solar neighbourhood’ by applying the mean shift clustering algorithm in energy–angular momentum space. Furthermore, a Fourier analysis of the space of orbital frequencies allows us to obtain accurate estimates of the time since accretion for approximately 30 per cent of the recovered satellites.