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

New data from the $\mathit{\text{Gaia}}$ 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 analyze the stellar density perturbations in the GD-1 stream and show that they cannot be due to known baryonic structures like 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 $\approx 10^{7}$to$10^9 \ M_{\rm{\odot}}$. We infer a total abundance of dark subhalos normalised to standard cold dark matter $n_{\rm sub}/n_{\rm sub, CDM} = 0.4 ^{+0.3}_{-0.2}$($68 %$), which corresponds to a mass fraction contained in the subhalos $f_{\mathrm{s}\mathrm{u}\mathrm{b}}=0.14_{-0.07}^{+0.11}\mathrm{\%}$, compatible with the predictions of hydrodynamical simulation of cold dark matter with baryons.