Stellar streams in the Gaia era

The hierarchical model of galaxy formation predicts that the Milky Way halo is populated by tidal debris of dwarf galaxies and globular clusters. Due to long dynamical times, debris from the lowest mass objects remains coherent as thin and dynamically cold stellar streams for billions of years. The Gaia mission, providing astrometry and spectrophotometry for billions of stars, has brought three fundamental changes to our view of cold stellar streams in the Milky Way. First, more than a hundred stellar streams have been discovered and characterized using Gaia data. This is an order of magnitude increase in the number of well-measured streams, thanks to Gaia’s capacity for identifying comoving groups of stars among the field Milky Way population. Second, Gaia data have revealed that density variations both along and across stellar streams are common. Dark-matter subhalos, as well as baryonic structures were theoretically predicted to form such features, but observational evidence for density variations was uncertain before Gaia. Third, stream kinematics are now widely available and have constrained the streams’ orbits and origins. Gaia has not only provided proper motions directly, but also enabled efficient spectroscopic follow-up of the proper-motion selected targets. These discoveries have established stellar streams as a dense web of sensitive gravitational tracers in the Milky Way halo. We expect the coming decade to bring a full mapping of the Galactic population of stellar streams, as well as develop numerical models that accurately explain their evolution within the Milky Way for a variety of cosmological models. Perhaps most excitingly, the comparison between the two will be able to reveal the presence of dark-matter subhalos below the threshold for galaxy formation ($\lesssim 10^6{M}_{\odot }$), and provide the most stringent test of the cold dark matter paradigm on small scales.