Binary neutron star merger offsets from their host galaxies. GW 170817 as a case study

TL;DR

This study models how the galactic potential and systemic kicks influence the offset distribution of binary neutron star mergers, using GW 170817 as a case study to understand their locations within host galaxies.

Contribution

It introduces a method to differentiate between kick prescriptions by analyzing BNS merger offsets in various galactic potentials, with GW 170817 as a key example.

Findings

GW 170817's location aligns with predictions for large or small kicks.

Large offsets in massive galaxies indicate large kicks.

Small offsets in dwarf galaxies suggest small kicks.

Abstract

Aims. The locations of binary neutron star (BNS) mergers within their host galaxies encode the systemic kicks that these systems received in the supernova aftermath. We investigate how the galactic potential and the systemic kicks shape the offset distribution of BNS mergers with a case study of GW 170817 and its host NGC 4993. Methods. We derived dynamical constraints on the host potential from integral field spectroscopy with Jeans anisotropic modelling. We evolved the trajectories of synthetic BNSs from the BPASS code in the galactic potential, using two different kick prescriptions to investigate how the observed offsets might differentiate between these two possibilities. The simulation was repeated after swapping the host potential with that of a dwarf galaxy, to test the effect of the potential on the offsets. Results. The location of GW 170817 is entirely consistent with our predictions, regardless of large or small kicks, because the strong potential of NGC 4993 is only diagnostic of very large kicks. In galaxies of similar or greater mass, large offsets can constrain large kicks, while small offsets do not provide much information. In an old dwarf galaxy, on the other hand, small offsets can constrain small kicks, while large offsets would prevent host association.