The observational signatures of supermassive black hole seeds

The origin and properties of the initial black hole seeds that grow to produce the observed population of accreting sources remain to be determined. It is a challenge to uniquely disentangle signatures of seeding from fuelling and dynamics that shapes the assembly history of growing black holes. To address this, we use a semi-analytic model developed to track the growth of supermassive black holes adopting multiple prescriptions for accretion. In contrast with earlier treatments, we explore the interplay between seeding models and two accretion modes. We find that signatures of the initial seeding do survive in the following observational probes: the black hole occupation fraction; contribution to the unresolved X-ray background; low-luminosity and high-redshift luminosity functions; and gravitational wave event signatures. We find that the behaviour of the low-mass end of the M-σ relation is dominated by uncertainties in the adopted accretion prescriptions and does not offer clear discrimination between seeding models. We make concrete predictions for future surveys, particularly for the Lynx X-ray surveyor and Laser Interferometer Space Antenna (LISA) mission, which will each provide different and yet strong constraints on the seed population. Black hole coalescences detected by LISA and high-redshift quasar luminosity functions observed by Lynx will offer the sharpest seeding discriminants. Although the signatures of the black hole seeding mechanism that persist remain linked to our understanding of black hole accretion and dynamics, we offer new insights on how these upcoming multiwavelength observations could be leveraged to effectively disentangle them.