Properties of dark subhaloes from gaps in tidal streams

Abstract

Cold or warm, the dark matter substructure spectrum must extend to objects with masses as low as 10⁷ M_⊙, according to the most recent Lyman α 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 complete set of properties (including the phase-space coordinates during the flyby) of dark perturbers with M > 10⁷ M_⊙, up to a 1D degeneracy between the mass and velocity.