# Associating Host Galaxy Candidates to Massive Black Hole Binaries resolved by Pulsar Timing Arrays

## TL;DR

This paper introduces a Bayesian method to identify potential host galaxies of gravitational wave sources from massive black hole binaries detected by pulsar timing arrays, significantly narrowing down candidate lists for follow-up observations.

## Contribution

The authors develop a novel ranking algorithm that combines GW amplitude dependence and galaxy correlations to efficiently identify likely host galaxies in large sky areas.

## Key findings

- 50% probability of host within 50 candidates for 100 sq. deg. localization
- 90% probability within 500 candidates, greatly reducing search space
- Method validated on realistic simulations with promising results

## Abstract

We propose a novel methodology to select host galaxy candidates of future pulsar timing array (PTA) detections of resolved gravitational waves (GWs) from massive black hole binaries (MBHBs). The method exploits the physical dependence of the GW amplitude on the MBHB chirp mass and distance to the observer, together with empirical MBH mass-host galaxy correlations, to rank potential host galaxies in the mass-redshift plane. This is coupled to a null-stream based likelihood evaluation of the GW amplitude and sky position in a Bayesian framework that assigns to each galaxy a probability of hosting the MBHB generating the GW signal. We test our algorithm on a set of realistic simulations coupling the likely properties of the first PTA resolved GW signal to synthetic all-sky galaxy maps. For a foreseeable PTA sky-localization precision of 100 squared degrees, we find that the GW source is hosted with 50% (90%) probability within a restricted number of <50 (<500) potential hosts. These figures are orders of magnitude smaller than the total number of galaxies within the PTA sky error-box, enabling extensive electromagnetic follow-up campaigns on a limited number of targets.