The Location and Environments of Neutron Star Mergers in an Evolving Universe

TL;DR

This paper uses cosmological simulations to study the locations and environments of neutron star mergers over cosmic time, providing insights into their distribution and the implications for electromagnetic follow-up observations.

Contribution

It introduces a novel combination of galaxy cluster evolution simulations with binary population synthesis to analyze merger environments across the universe.

Findings

Merger sites are often in low-density environments at higher redshifts.

Environmental densities of mergers vary significantly with redshift.

Model sensitivity affects predicted merger location distributions.

Abstract

The simultaneous detection of gravitational and electromagnetic waves from a binary neutron star merger has both solidified the link between neutron star mergers and short-duration gamma-ray bursts (GRBs) and demonstrated the ability of astronomers to follow-up the gravitational wave detection to place constraints on the ejecta from these mergers as well as the nature of the GRB engine and its surroundings. As the sensitivity of aLIGO and VIRGO increases, it is likely that a growing number of such detections will occur in the next few years, leading to a sufficiently-large number of events to constrain the populations of these GRB events. While long-duration GRBs originate from massive stars and thus are located near their stellar nurseries, binary neutron stars may merge on much longer timescales, and thus may have had time to migrate appreciably. The strength and character of the electromagnetic afterglow emission of binary neutron star mergers is a sensitive function of the circum-merger environment. Though the explosion sites of short GRBs have been explored in the literature, the question has yet to be fully addressed in its cosmological context. We present cosmological simulations following the evolution of a galaxy cluster including star formation combined with binary population synthesis models to self-consistently track the locations and environmental gas densities of compact binary merger sites throughout the cosmic web. We present probability distributions for densities as a function of redshift and discuss model sensitivity to population synthesis model assumptions.