Evolving SMBH-Bulge Relation Explains Gravitational Wave Background Amplitude

The research addresses the discrepancy between the observed gravitational wave background (GWB) spectrum and model predictions, which shows the observed spectrum being higher in amplitude by a factor of 2-3. The study proposes that an evolving intrinsic scatter in the supermassive black hole (SMBH) to bulge mass relation (Mbh-Mbulge) can explain this discrepancy. By implementing an intrinsic scatter that increases at higher redshift but aligns with local observations, the amplitude of GWB models increases to be consistent with the low-frequency end of the GWB spectrum. The evolving scatter preferentially increases the number density of the most massive SMBHs, minimizing the strength of the low-frequency turnover in the GWB spectrum. The models suggest that the Mbh-Mbulge relation we observe today is not universal throughout cosmic time, implying a diversity of seeding models and growth mechanisms in the early stages of SMBH-galaxy evolution. The results indicate that the Mbh-Mbulge relation which best describes the available GWB and electromagnetic data sets has intrinsic scatter which evolves as epsilon(z) = epsilon_0 + (0.56 +/- 0.4) log10(1 + z) and normalization which evolves as alpha(z) = alpha_0 (1 + z)^(0.84 +/- 0.35). This research highlights the complex interplay between SMBHs and their host galaxies, suggesting that their co-evolution is not a static process but rather one that evolves significantly over cosmic time. Further research is needed to refine these models and to fully understand the implications of an evolving SMBH-bulge relation for galaxy formation and SMBH growth.

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