Contribution of Neutron Star Mergers to the R-process Chemical Evolution in the Hierarchical Galaxy Formation

The main astronomical source of r-process elements has not yet been identified. One plausible site is neutron star mergers (NSMs), but from perspective of the Galactic chemical evolution, it has been pointed out that NSMs cannot reproduce the observed r-process abundance distribution of metal-poor stars at [Fe/H] $< -3$. Recently, Tsujimoto & Shigeyama (2014) pointed out that NSM ejecta can spread into much larger volume than ejecta from a supernova. We re-examine the enrichment of r-process elements by NSMs considering this difference in propagation using the chemical evolution model under the hierarchical galaxy formation. The observed r-process enhanced stars around [Fe/H] $\sim -3$ are reproduced if the star formation efficiency is lower for low-mass galaxies under a realistic delay time distribution for NSMs. We show that a significant fraction of NSM ejecta escape from its host proto-galaxy to pollute intergalactic matter and other proto-galaxies. The propagation of r-process elements over proto-galaxies changes the abundance distribution at [Fe/H] $< -3$ and obtains distribution compatible with observations of the Milky Way halo stars. In particular, the pre-enrichment of intergalactic medium explains the observed scarcity of EMP stars without Ba and abundance distribution of r-process elements at [Fe/H] $\lesssim -3.5$.