Speaker
Description
Active galactic nuclei (AGN) disks provide a dense gaseous environment where stellar-mass black holes (BHs) can efficiently form binaries and undergo hierarchical mergers, potentially producing the high-mass and high-spin binary black holes (BBHs) observed by the LIGO–Virgo–KAGRA (LVK) collaboration.
We present an updated semi-analytical population framework that follows the dynamical evolution of BH populations embedded in AGN disks, including gas capture, migration, binary formation, gas hardening, and dynamical binary–single interactions. Building on previous work, we now incorporate the cosmological evolution of AGN populations by combining observational constraints on the low-redshift AGN luminosity function with high-redshift AGN populations predicted by cosmological simulations, in order to compute the redshift-dependent BBH merger rate from this channel.
We find that the predicted merger rate and hierarchical merger fraction depend strongly on the cosmological model chosen and on the disk viscosity parameter $\alpha$, which regulates migration and hardening rates within the disk. Variations in $\alpha$ can change the observable BBH merger rate from the AGN channel by orders of magnitude, making gravitational-wave observations directly sensitive to AGN disk microphysics. The binary formation pathways in AGN disks also imprint characteristic spin and eccentricity signatures compared to other formation channels.
These models therefore yield testable predictions for the mass spectrum and spin distribution of BBHs detectable by current and future gravitational-wave observatories. Future observations with the Einstein Telescope (ET) will therefore offer a new probe of AGN disk physics and the growth of supermassive BHs across cosmic time.