Patrick Horlaville

Bishop’s University

Patrick Horlaville is a second-year Master’s student at Bishop’s University working under the supervision of Dr. John Ruan. His research project explores how we can identify the host galaxies of supermassive black hole binaries that will be detected with gravitational waves in the near future with radio telescopes on Earth known as Pulsar Timing Arrays (PTAs). Supermassive black holes live in the center of most massive galaxies and can form binary systems if their host galaxies merge with each other. At close enough separations, these binary systems emit low-frequency (nHz) gravitational waves detectable by PTAs. While recent detections point towards an abundant population of supermassive black hole binaries in the local Universe, we have yet to directly detect and localize individual systems, which would allow us to conduct follow-up electromagnetic observations. This in turn would shed light on the mechanisms that can bring supermassive black holes to form binaries and eventually merge, which remain largely unconstrained.

Currently, there are no reliable methods to localize the host galaxy of supermassive black hole binaries using gravitational wave observations from PTAs. However, recent work using cosmological simulations suggests that the host galaxies of supermassive black hole binaries have unique stellar kinematics. Notably, because we expect the binary systems that will be detected in gravitational waves to be massive and close by, they should live in local massive galaxies, which have for the most part already been observed by integral field unit (IFU) galaxy surveys. For his Master’s research project, Patrick is searching for those characteristic stellar kinematic signatures in archival IFU galaxy surveys to identify supermassive black hole binary host galaxy candidates, which will speed up the gravitational wave search for PTAs. When PTAs identify the gravitational wave signature of a supermassive black hole binary system and its host galaxy is localized, we will be able to investigate fundamental questions related to the co-evolution of supermassive black holes and host galaxy environments.

By searching through the MASSIVE, ATLAS3D and CALIFA galaxy surveys, Patrick’s objective is to identify the galaxies that are the most likely to host supermassive black hole binaries whose gravitational waves will be detected by PTAs in the near future. On the x-axis, galaxies with a high LDA score display the strongest kinematic signatures of supermassive black hole binary host galaxies. On the y-axis, galaxies with a high log(h0) host the most massive and closest supermassive black hole systems, which are the ones most likely to be detected with gravitational waves by PTAs if they exist in a binary. Therefore, galaxies located in the top right corner of the plot both display the stellar kinematic of supermassive black hole binary hosts, and their supermassive black holes are massive and close, making them ideal candidates for the host galaxies of supermassive black hole binaries that will be detected by PTAs.