The Distribution of Coalescing Compact Binaries in the Local Universe: Prospects for Gravitational-Wave Observations

TL;DR

This paper models the spatial distribution of merging compact binaries in the local universe using dark matter simulations, highlighting how their velocities and offsets influence gravitational wave detection and electromagnetic follow-up strategies.

Contribution

It provides a detailed analysis of the distribution and sky localization of coalescing binaries considering natal kicks, informing observational strategies for GW detectors.

Findings

Mergers occur up to a few Mpc from host halos due to natal kicks.

Large kicks increase the isotropy of GW source sky distributions.

Differences in kick models could be distinguished by GW observations.

Abstract

Merging compact binaries are the most viable and best studied candidates for gravitational wave (GW) detection by the fully operational network of ground-based observatories. In anticipation of the first detections, the expected distribution of GW sources in the local universe is of considerable interest. Here we investigate the full phase space distribution of coalescing compact binaries at $z = 0$ using dark matter simulations of structure formation. The fact that these binary systems acquire large barycentric velocities at birth ("kicks") results in merger site distributions that are more diffusely distributed with respect to their putative hosts, with mergers occurring out to distances of a few Mpc from the host halo. Redshift estimates based solely on the nearest galaxy in projection can, as a result, be inaccurate. On the other hand, large offsets from the host galaxy could aid the detection of faint optical counterparts and should be considered when designing strategies for follow-up observations. The degree of isotropy in the projected sky distributions of GW sources is found to be augmented with increasing kick velocity and to be severely enhanced if progenitor systems possess large kicks as inferred from the known population of pulsars and double compact binaries. Even in the absence of observed electromagnetic counterparts, the differences in sky distributions of binaries produced by disparate kick-velocity models could be discerned by GW observatories, within the expected accuracies and detection rates of advanced LIGO--in particular with the addition of more interferometers.