Motivated by recent findings that the nHz signal of Pulsar Timing Arrays (PTAs) may be dominated by supermassive black hole (SMBH) binaries ($M\gtrsim {10}^9{M}_{\odot }$), and high redshift quasar observations revealing unexpectedly massive SMBHs, we calculate the implications of early seeded SMBHs for the PTA signal. As an application, we explore two prominent scenarios of high-$z$ SMBH seeding mechanisms: direct collapse black holes (DCBHs) and collapse of Dark Stars. We show that Dark Star seeded SMBHs with comoving seed number density of $\mathcal{O}({10}^{-3}){\mathrm{Mpc}}^{-3}$ can be the dominant contributor to the PTA signal while the DCBH channel may contribute subdominantly. We also suggest ways to place an upper bound on the seed number density.

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• Gravitational wave sources
• Particle astrophysics