Constraining Black Hole Growth History With Population Spin Measurements

Constraining the primary growth channel of supermassive black holes (SMBH) remains one the most actively debated questions in the context of cosmological structure formation. Owing to the connection between SMBH spin parameter evolution and accretion-/merger-dominated black hole growth, population spin measurements offer a rare observational window into SMBH cosmic assembly history. Here, we investigate the capability of statistical samples of spin parameter measurements for distinguishing primary SMBH growth channels, based on their mass vs spin distributions predicted by the Horizon-AGN cosmological simulation. By considering realistic BAT AGN Spectroscopic Survey targets, known observational biases and intrinsic uncertainty associated with relativistic reflection models, we show how future broadband X-ray studies will confidently constrain the primary mode of SMBH growth in the observable Universe, a feat beyond the reach of current measurements.