Binary neutron star merger offsets from their host galaxies

GW 170817 as a case study

¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
² Astrophysics sub-Department, Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
³ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
⁴ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁵ European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands

^⋆ Corresponding author; nicola.gaspari@live.it

Received: 29 May 2024
Accepted: 21 October 2024

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.