Shining Light on the Hosts of the Nano-Hertz Gravitational Wave Sources: A Theoretical Perspective

TL;DR

This paper uses cosmological simulations to explore the properties of galaxies hosting supermassive black hole binaries that generate nano-Hertz gravitational waves, aiming to connect theoretical models with observational data.

Contribution

It presents the first analysis linking host galaxy properties from simulations to nano-Hertz GW sources, enhancing understanding of galaxy and SMBH co-evolution.

Findings

High chirp mass sources are mainly in low-redshift, early-type galaxies.

Host galaxy mass correlates with SMBH binary mass.

Sources are often in galaxy groups with recent mergers.

Abstract

The formation of supermassive black holes (SMBHs) in the Universe and its role in the properties of the galaxies is one of the open questions in astrophysics and cosmology. Though, traditionally, electromagnetic waves have been instrumental in direct measurements of SMBHs, significantly influencing our comprehension of galaxy formation, gravitational waves (GW) bring an independent avenue to detect numerous binary SMBHs in the observable Universe in the nano-Hertz range using the pulsar timing array observation. This brings a new way to understand the connection between the formation of binary SMBHs and galaxy formation if we can connect theoretical models with multi-messenger observations namely GW data and galaxy surveys. Along these lines, we present here the first paper on this series based on {\sc Romulus25} cosmological simulation on the properties of the host galaxies of SMBHs and propose on how this can be used to connect with observations of nano-Hertz GW signal and galaxy surveys. We show that the most dominant contribution to the background will arise from sources with high chirp masses which are likely to reside in low redshift early-type galaxies with high stellar mass, largely old stellar population, and low star formation rate, and that reside at centers of galaxy groups and manifest evidence of recent mergers. The masses of the sources show a correlation with the halo mass and stellar mass of the host galaxies. This theoretical study will help in understanding the host properties of the GW sources and can help in establishing a connection with observations.