Properties of Dark Subhaloes from Gaps in Tidal Streams
  Cold or Warm, the Dark Matter substructure spectrum must extend to objects
with masses as low as $10^7 M_\odot$, according to the most recent
Lyman-$\alpha$ measurements. Around a Milky Way-like galaxy, more than a
thousand of these subhaloes will not be able to form stars but are dense enough
to survive even deep down in the potential well of their host. There, within
the stellar halo, these dark pellets will bombard tidal streams as they travel
around the Galaxy, causing small but recognizable damage to the stream density
distribution. The detection and characterization of these stream ruptures will
allow us to constrain the details of the subhalo-stream interaction. In this
work, for the first time, we will demonstrate how the properties of a subhalo,
most importantly its mass and size, can be reliably inferred from the gap it
produces in a tidal stream. For a range of realistic observational setups,
mimicking e.g. SDSS, DES, Gaia and LSST data, we find that it is possible to
measure the {\it complete set} of properties (including the phase-space
coordinates during the flyby) of dark perturbers with $M>10^7 M_\odot$, up to a
1d degeneracy between the mass and velocity.
