Abstract
We analyze the distribution of stars along the GD-1 stream with a combination
of data from the ${\it Gaia}$ satellite and the Pan-STARRS survey, and we show
that the population of subhalos predicted by the cold dark matter paradigm are
necessary and sufficient to explain the perturbations observed in the linear
density of stars. This allows us to set novel constraints on alternative dark
matter scenarios that predict a suppression of the subhalo mass function on
scales smaller than the mass of dwarf galaxies. A combined analysis of the
density perturbations in the GD-1 and Pal 5 streams leads to a $95\%$ lower
limit on the mass of warm dark matter thermal relics $m_{\rm WDM}>4.6$ keV;
adding dwarf satellite counts strengthens this to $m_{\rm WDM}>6.3$ keV.