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Recent progress of nucleosynthesis work as well as the discovery of akilonova associated with the gravitational-wave source GW170817 indicates thatneutron star mergers (NSM) can be a site of the r-process. Several studies ofgalactic chemical evolution, however, have pointed out inconsistencies betweenthis idea and the observed stellar abundance signatures in the Milky Way: (a)the presence of Eu at low (halo) metallicity and (b) the descending trend ofEu/Fe at high (disc) metallicity. In this study, we explore the galacticchemical evolution of the Milky Way's halo, disc and satellite dwarf galaxies.Particular attention is payed to the forms of delay-time distributions for bothtype Ia supernovae (SN Ia) and NSMs. The Galactic halo is modeled as anensemble of independently evolving building-block galaxies with differentmasses. The single building blocks as well as the disc and satellite dwarfs aretreated as well-mixed one-zone systems. Our results indicate that theaforementioned inconsistencies can be resolved and thus NSMs can be the uniquer-process site in the Milky Way, provided that the delay-time distributionssatisfy the following conditions: (i) a long delay (~1 Gyr) for the appearanceof the first SN Ia (or a slow early increase of its number) and (ii) anadditional early component providing >~ 50% of all NSMs with a delay of ~0.1Gyr. In our model, r-process-enhanced and r-process-deficient stars in the haloappear to have originated from ultra-faint dwarf-sized and massive buildingblocks, respectively. Our results also imply that the natal kicks of binaryneutron stars have a little impact on the evolution of Eu in the disc.

Neutron star mergers as the astrophysical site of the r-process in the Milky Way and its satellite galaxies