Host Galaxy Demographics Of Individually Detectable Supermassive Black-hole Binaries with Pulsar Timing Arrays

TL;DR

This study uses cosmological simulations to identify the properties of host galaxies of supermassive black-hole binaries detectable by pulsar timing arrays, revealing they are typically bright, elliptical galaxies with higher metallicity.

Contribution

It provides the first detailed analysis of host galaxy characteristics of detectable supermassive black-hole binaries using cosmological simulations and GW detection modeling.

Findings

Detectable systems are hosted by galaxies with higher metallicity (~2 Z_sun).

Most detectable binaries are in bright, red, elliptical galaxies.

Host galaxy properties differ significantly from the general galaxy population.

Abstract

Massive black hole binaries (MBHBs) produce gravitational waves (GWs) that are detectable with pulsar timing arrays. We determine the properties of the host galaxies of simulated MBHBs at the time they are producing detectable GW signals. The population of MBHB systems we evaluate is from the \textit{Illustris} cosmological simulations taken in tandem with post processing semi-analytic models of environmental factors in the evolution of binaries. Upon evolving to the GW frequency regime accessible by pulsar timing arrays, we calculate the detection probability of each system using a variety of different values for pulsar noise characteristics in a plausible near-future International Pulsar Timing Array dataset. We find that detectable systems have host galaxies that are clearly distinct from the overall binary population and from most galaxies in general. With conservative noise factors, we find that host stellar metallicity, for example, peaks at $\sim 2 Z_\odot$ as opposed to the total population of galaxies which peaks at $\sim 0.6 Z_\odot$. Additionally, the most detectable systems are much brighter in magnitude and more red in color than the overall population, indicating their likely identity as large ellipticals with diminished star formation. These results can be used to develop effective search strategies for identifying host galaxies and electromagnetic counterparts following GW detection by pulsar timing arrays.