Most, if not all, massive galaxies host a supermassive black hole (SMBH) in their nucleus. Yet the formation and evolution of these SMBHs remains one of the key open questions in astrophysics. This has gained renewed attention as, at low redshift, PTAs have recently detected a gravitational wave background, however this may be in tension with the demography of local SMBHs. While at high redshift, JWST has significantly extended the observable parameter space, revealing a diverse population of active galactic nuclei (AGN) which, at face value, appear both more massive and more numerous than the pre-JWST expectations.
Motivated by these findings, we present a data-driven investigation of the coevolution of SMBHs and their host galaxies across cosmic time (z = 10 - 0) using the semi-empirical model DECODE. In this model, the growth of galaxies is associated to that of its host halo via abundance matching between the star-formation rate and halo-accretion rate, whereas BHs are grown via the observed accretion rate distribution, with mergers included in accordance with the dark matter merger trees. Thereby offering physical insights, while minimising the number of assumptions and free parameters.
At high z, we demonstrate that current estimates of the AGN luminosity function can reproduce the most massive AGN observed by JWST at z ~ 6 – 8 and predicts a mean SMBH mass – stellar mass relation at z = 6 that lies below the AGN observed by JWST, but is consistent with them being the tail of the underlying distribution.
At intermediate z, we find that the BH-galaxy scaling relations are near constant with redshift, but that the intrinsic scatter decreases with both cosmic time and mass. Furthermore, this evolution of the intrinsic scatter is driven primarily by accretion, with mergers playing a minor role.
At low z, we find that local SMBH mass function is primarily driven by accretion, with mergers only meaningfully contributing at high masses, and that this lies below the mass function claimed to be consistent with the PTA measurements. Thus, corroborating the tension previously identified within a distinct data-driven framework.
In the near future, we will further extend DECODE to predict the gravitational wave signal in both the PTA and LISA bands.
09/06/2026
Speaker: Daniel Roberts - University of Southampton
June 9th at 11:00 CEST in Aula Gratton