  Elements heavier than zinc are synthesized through the (r)apid and (s)low
neutron-capture processes. The main site of production of the r-process
elements (such as europium) has been debated for nearly 60 years. Initial
studies of chemical abundance trends in old Milky Way halo stars suggested
continual r-process production, in sites like core-collapse supernovae. But
evidence from the local Universe favors r-process production mainly during rare
events, such as neutron star mergers. The appearance of a europium abundance
plateau in some dwarf spheroidal galaxies has been suggested as evidence for
rare r-process enrichment in the early Universe, but only under the assumption
of no gas accretion into the dwarf galaxies. Cosmologically motivated gas
accretion favors continual r-process enrichment in these systems. Furthermore,
the universal r-process pattern has not been cleanly identified in dwarf
spheroidals. The smaller, chemically simpler, and more ancient ultra-faint
dwarf galaxies assembled shortly after the first stars formed, and are ideal
systems with which to study nucleosynthesis events such as the r-process.
Reticulum II is one such galaxy. The abundances of non-neutron-capture elements
in this galaxy (and others like it) are similar to those of other old stars.
Here, we report that seven of nine stars in Reticulum II observed with
high-resolution spectroscopy show strong enhancements in heavy neutron-capture
elements, with abundances that follow the universal r-process pattern above
barium. The enhancement in this "r-process galaxy" is 2-3 orders of magnitude
higher than that detected in any other ultra-faint dwarf galaxy. This implies
that a single rare event produced the r-process material in Reticulum II. The
r-process yield and event rate are incompatible with ordinary core-collapse
supernovae, but consistent with other possible sites, such as neutron star
mergers.
