Signatures of Massive Black Hole Merger Host Galaxies from Cosmological Simulations II: Unique Stellar Kinematics in Integral Field Unit Spectroscopy
Jaeden Bardati, John J. Ruan, Daryl Haggard, Michael Tremmel, Patrick Horlaville
TL;DR
This study extends the identification of MBH merger host galaxies from imaging morphologies to stellar kinematics observable with integral field unit spectroscopy. Using the Romulus25 cosmological simulation, the authors generate synthetic 3D spectral datacubes for MBH merger hosts and a matched control sample, derive 2D stellar kinematic maps via full-spectrum fitting, and extract 12 kinematic features. A linear discriminant analysis (LDA) trained on these features achieves high accuracy, up to >85% for mergers with high chirp mass ($M_{chirp}\gtrsim 10^{8.2}\,M_\odot$) and high mass ratio ($M_2/M_1>0.5$); the strongest discriminants are slower rotation ($\lambda_{R_e}$) and larger misalignment between kinematic and photometric axes ($\Delta\mathrm{PA}$). The results support a coherent picture in which MBH binaries preferentially reside in massive, bulge-dominated galaxies formed by major mergers, and suggest IFU spectroscopy as a valuable tool for follow-up of gravitational-wave detections, particularly for nearby, massive systems detectable by PTAs. When combined with imaging-based morphology, these kinematic signatures can significantly narrow host-galaxy candidates within GW localization volumes, enabling robust multi-messenger studies of MBH growth and galaxy evolution.
Abstract
Secure methods for identifying the host galaxies of individual massive black hole (MBH) binaries and mergers detected by gravitational wave experiments such as LISA and Pulsar Timing Arrays are currently lacking, but will be critical to a variety of science goals. Recently in Bardati et al. (2024, Paper I), we used the Romulus25 cosmological simulation to show that MBH merger host galaxies have unique morphologies in imaging, due to their stronger bulges. Here, we use the same sample of simulated MBH merger host galaxies to investigate their stellar kinematics, as probed by optical integral field unit (IFU) spectroscopy. We perform stellar population synthesis and dust radiative transfer to generate synthetic 3D optical spectral datacubes of each simulated galaxy, and produce mock stellar kinematic maps. Based on a linear discriminant analysis of a combination of kinematic parameters derived from these maps, we show that this approach can identify MBH binary and merger host galaxies with accuracies that increase with chirp mass and mass ratio. For mergers with high chirp masses (>10^8.2 Msun) and high mass ratios (>0.5), the accuracies reach >85%, and their host galaxies are uniquely characterized by slower rotation and stronger stellar kinematic misalignments. These kinematic properties are commonly associated with massive early-type galaxies that have experienced major mergers, and naturally act as signposts for MBH binaries and mergers with high chirp masses and mass ratios. These results suggest that IFU spectroscopy should also play a role in telescope follow-up of future MBH binaries and mergers detected in gravitational waves.