Evidence of a population of dark subhaloes from Gaia and Pan-STARRS observations of the GD-1 stream

New data from the Gaia satellite, when combined with accurate photometry from the Pan-STARRS survey, allow us to accurately estimate the properties of the GD-1 stream. Here, we analyse the stellar density variations in the GD-1 stream and show that they cannot be due to known baryonic structures such as giant molecular clouds, globular clusters, or the Milky Way’s bar or spiral arms. A joint analysis of the GD-1 and Pal 5 streams instead requires a population of dark substructures with masses ≈107–10⁹ \ M. We infer a total abundance of dark subhaloes normalized to standard cold dark matter nsub/nsub, CDM = 0.4 +0.3-0.2 (68 per cent), which corresponds to a mass fraction contained in the subhaloes fsub = 0.14 +0.11-0.07 \ per\ cent, compatible with the predictions of hydrodynamical simulation of cold dark matter with baryons.