Morphological evolution of supermassive black hole merger hosts and multimessenger signatures

TL;DR

This study uses the Illustris simulation to analyze the properties and host galaxy characteristics of supermassive black hole mergers, predicting their gravitational wave signatures and electromagnetic counterparts for upcoming detectors like LISA and PTAs.

Contribution

It provides new insights into the galaxy and black hole merger rates, host galaxy features, and the impact of merger delays on detection prospects for future gravitational wave observatories.

Findings

Black hole mergers occur in typical galaxies along the $M_{\rm{BH}}-M_*$ relation.

Galaxy mergers can trigger increased star formation in low-mass black hole hosts.

Incorporating merger delay times shifts detection rates from LISA to PTAs.

Abstract

With projects such as Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays expected to detect gravitational waves from supermassive black hole mergers in the near future, it is key that we understand what we expect those detections to be, and maximize what we can learn from them. To address this, we study the mergers of supermassive black holes in the Illustris simulation, the overall rate of mergers, and the correlation between merging black holes and their host galaxies. We find that these mergers occur in typical galaxies along the $M_{\rm{BH}}-M_*$ relation, and that between LISA and PTAs we expect to probe the full range of galaxy masses. As galaxy mergers can trigger increased star formation, we find that galaxies hosting low-mass black hole mergers tend to show a slight increase in star formation rates compared to a mass-matched sample. However, high-mass merger hosts have typical star formation rates, due to a combination of low gas fractions and powerful AGN feedback. Although minor black hole mergers do not correlate with disturbed morphologies, major mergers (especially at high-masses) tend to show morphological evidence of recent galaxy mergers which survives for ~500 Myr. This is on the same scale as the infall/hardening time of the merging black holes, suggesting that electromagnetic followups to gravitational wave signals may not be able to observe this correlation. We further find that incorporating a realistic timescale delay for the black hole mergers could shift the distribution of merger masses toward higher-masses, decreasing the rate of LISA detections while increasing the rate of PTA detections.