Constraining Supermassive Black Hole Evolution with Pulsar Timing Array-Informed Mass Functions

Recent pulsar timing array (PTA) observations of the stochastic gravitational wave background reveal a signal stronger than predicted by standard models of supermassive black hole (SMBH) populations, suggesting that the high-mass end may be underestimated. This thesis investigates how modifying the local SMBH mass function to match PTA data impacts the inferred history of SMBH growth. Using a continuity equation framework and constraints from the quasar luminosity function, we model SMBH evolution across cosmic time and constrain the active duty cycle through Markov Chain Monte Carlo methods. The analysis provides strong evidence for AGN downsizing, indicating that the most massive black holes formed and accreted earlier in the universe.