Measuring the Delay Time Distribution of Binary Neutron Stars. I. Through Scaling Relations of the Host Galaxies of Gravitational-wave Events

The delay time distribution of (DTD) of binary neutron stars (BNSs) remains poorly constrained, mainly by the small known population of Galactic binaries, the properties of short gamma-ray burst host galaxies, and inferences from r-process enrichment. In the new era of BNS merger detections through gravitational waves (GWs), a new route to the DTD is the demographics of the host galaxies, localized through associated electromagnetic counterparts. This approach takes advantage of the correlation between star formation history (SFH) and galaxy mass, such that the convolution of the SFH and DTD impacts the BNS merger rate as a function of galaxy mass. Here we quantify this approach for a power-law DTD governed by two parameters: the power-law index (Γ) and a minimum delay time (t _min). Under the reasonable assumption that electromagnetic counterparts are likely only detectable in the local universe, accessible by the current generation of GW detectors, we study how many host galaxies at z ∼ 0 are required to constrain the DTD parameters. We find that the DTD is mainly imprinted in the statistics of massive galaxies (stellar mass of M _* ≳ 10^10.5 M _⊙, comparable to the host galaxy of GW170817). Taking account of relevant uncertainties we find that { \mathcal O }({10}³) host galaxies are required to constrain the DTD; for a fixed value of t _min, as done in previous analyses of the DTD, { \mathcal O }({10}²) host galaxies will suffice. Such a sample might become available within the next two decades, prior to the advent of third-generation GW detectors.