Uncovering cold dark matter halo substructure with tidal streams

Models for the formation and growth of structure in a cold-dark-matter-dominated universe predict that galaxy haloes should contain significant substructure. Studies of the Milky Way, however, have yet to identify the expected few hundred subhaloes with masses greater than about 10(6) M-circle dot . Here we propose a test for the presence of subhaloes in the haloes of galaxies. We show that the structure of the tidal tails of ancient globular clusters is very sensitive to heating by repeated close encounters with the massive dark subhaloes. We discuss the detection of such an effect in the context of the next generation of astrometric missions, and conclude that it should be easily detectable with the GAIA data set. The finding of a single extended cold stellar stream from a globular cluster would support alternative theories, such as self-interacting dark matter, that give rise to smoother haloes.