Cosmic Cartography II: completing galaxy catalogs for gravitational-wave cosmology

TL;DR

This paper introduces a novel method to reconstruct the true galaxy distribution, including spatial and magnitude information, to improve the accuracy of gravitational-wave based cosmological measurements, specifically the Hubble constant.

Contribution

It develops an advanced reconstruction technique that accounts for galaxy distribution biases, improving the robustness of dark siren cosmology analyses.

Findings

Improved galaxy field reconstruction respects large-scale spatial correlations.

Enhanced prior for host galaxy localization improves H_0 inference.

Validated method on Millennium simulation data.

Abstract

The dark siren method exploits the complementarity between gravitational-wave binary coalescence signals and galaxy catalogs originating from the same regions of space. However, all galaxy catalogs are incomplete, i.e. they only include a subset of all galaxies, typically being biased towards the bright end of the luminosity distribution. This sub-selection systematically affects the dark siren inference of the Hubble constant $H_0$, so a completeness relation has to be introduced that accounts for the missing objects. In the literature it is standard to assume that the missing galaxies are uniformly distributed across the sky and that the galaxy magnitude distribution is known. In this work we develop a novel method which improves upon these assumptions and reconstructs the underlying true galaxy field, respecting the spatial correlation of galaxies on large scales. In our method the true magnitude distribution of galaxies is inferred alongside the spatial galaxy distribution. Our method results in an improved three-dimensional prior in redshift and sky position for the host galaxy of a GW event, which is expected to make the resulting $H_0$ posterior more robust. Building on our previous work, we make a number of improvements, and validate our method on simulated data based on the Millennium simulation. The inference results can be reproduced through our publicly available code base light.