Black holes (BHs) as massive as 10^9 solar masses has been discovered at redshift z > 6, that is within one billion years after the big bang. It is believed that some seed black holes with large initial masses formed at very early times, which then grow to the supermassive black holes we observe at z ∼ 6. First stars (Pop III) remnants are considered to be one of the promising pathways to form such seed BHs. We study the dynamics of seed BHs formed out of Pop III star remnants through cosmological simulation. We evolve a representative volume across 1Mpc in comoving coordinates, and write 1000 outputs every ∼0.5 Myr. We then extract all seed BHs in each output and investigate the distribution and evolution of their orbital properties from redshift z ∼ 17 to z ∼ 9, which are crucial to their growth and merger rate. We show that most of seed BHs in halos are gravitationally bounded, and a significant fraction of them have orbits with short semi-major axes and high eccentricity. They are prime candidates for black hole mergers as they are the ones that will venture closest to the halo center and any massive black hole that might exist in the (proto-)galaxy center. We attempt to estimate the merger rate of BHs by calculating the loss rate of angular momentum, but find that this approach is not suitable for our simulation data.
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