  The origins of most stellar streams in the Milky Way are unknown. With
improved proper motions provided by Gaia EDR3, we show that the orbits of 23
Galactic stellar streams are highly clustered in orbital phase space. Based on
their energies and angular momenta, most streams in our sample can plausibly be
associated with a specific (disrupted) dwarf galaxy host that brought them into
the Milky Way. For eight streams we also identify likely globular cluster
progenitors (four of these associations are reported here for the first time).
Some of these stream progenitors are surprisingly far apart, displaced from
their tidal debris by a few to tens of degrees. We identify stellar streams
that appear spatially distinct, but whose similar orbits indicate they likely
originate from the same progenitor. If confirmed as physical discontinuities,
they will provide strong constraints on the mass-loss from the progenitor. The
nearly universal ex-situ origin of existing stellar streams makes them valuable
tracers of galaxy mergers and dynamical friction within the Galactic halo.
Their phase-space clustering can be leveraged to construct a precise global map
of dark matter in the Milky Way, while their internal structure may hold clues
to the small-scale structure of dark matter in their original host galaxies.
