PhD. Thesis Defense — Chao Shi — The Dynamics of Black Holes in First Galaxies

Abstract:

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 hole 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.