Predicted rates of merging neutron stars in galaxies

TL;DR

This paper models neutron star merger rates across galaxy types and cosmic time, predicting kilonova event rates for future observations and comparing different delay time distributions and cosmological scenarios.

Contribution

It provides new predictions for neutron star merger rates in various galaxy types and cosmic scenarios, incorporating different delay time distributions and their implications for observations.

Findings

The observed neutron star merger rate in the Milky Way is well reproduced by models with short or distributed delays.

The cosmic neutron star merger rate aligns with short Gamma Ray Burst rates in hierarchical cosmological models.

Future kilonova observations in ellipticals can distinguish between delay time distributions and cosmological scenarios.

Abstract

In this work, we compute rates of merging neutron stars (MNS) in galaxies of different morphological type, as well as the cosmic MNS rate in a unitary volume of the Universe adopting different cosmological scenarios. Our aim is to provide predictions of kilonova rates for future observations both at low and high redshift. In the adopted galaxy models, we take into account the production of r-process elements either by MNS or core-collapse supernovae. In computing the MNS rates we adopt either a constant total time delay for merging (10 Myr) or a distribution function of such delays. Our main conclusions are: i) the observed present time MNS rate in our Galaxy is well reproduced either with a constant time delay or a distribution function $\propto t^{-1}$. The [Eu/Fe] vs. [Fe/H] relation in the Milky Way can be well reproduced with only MNS, if the time delay is short and constant. If the distribution function of delays is adopted, core-collapse supernovae as are also required. ii) The present time cosmic MNS rate can be well reproduced in any cosmological scenario, either pure luminosity evolution or a typical hierarchical one, and spirals are the main contributors to it. iii) The spirals are the major contributors to the cosmic MNS at all redshifts in hierarchical scenarios. In the pure luminosity evolution scenario, the spirals are the major contributors locally, whereas at high redshift ellipticals dominate. iv) The predicted cosmic MNS rate well agrees with the cosmic rate of short Gamma Ray Bursts if the distribution function of delays is adopted, in a cosmological hierarchical scenario observationally derived. v) Future observations of Kilonovae in ellipticals will allow to disentangle among constant or a distribution of time delays as well as among different cosmological scenarios.