Reconstructing PTA measurements via early seeding of supermassive black holes

Abstract

The growing evidence of supermassive black holes (SMBHs) being present in the early Universe poses challenges to their traditional formation pathways. Separately, studies suggest that merging SMBH binaries with total masses $\gtrsim 10^9 M_\odot$ could be the primary sources of the nanohertz gravitational wave background detected by the Pulsar Timing Arrays (PTAs). Owing to their extreme masses, these SMBHs have a higher probability of forming earlier than their lower-mass counterparts. In this work, we provide a formalism to calculate the implications of early seeded SMBHs for the PTA signal. As an application, we explore the two most prominent scenarios of high-$z$ SMBHs seeding mechanisms: direct collapse black holes (DCBHs) and collapse of supermassive dark stars (SMDSs). We show that SMDS-seeded SMBHs, with comoving seed number density of $\mathcal{O}(10^{-3}) \, {\rm Mpc}^{-3}$ can be the dominant contributor to the PTA signal, with mass $> 10^9 M_\odot$ binaries acquiring a peak merger rate at $z< 1$. By contrast, SMBHs seeded via the DCBH channel may contribute only sub-dominantly unless produced with a much larger number density than implied by numerical simulations. Our analysis further suggests that the seed number density should be $< \mathcal{O}(10^{-1}) \, {\rm Mpc}^{-3}$ in order to not over-predict the expected PTA signal.