Seeding High Redshift QSOs by Collisional Runaway in Primordial Star
  Clusters
  We study how runaway stellar collisions in high-redshift, metal-poor star
clusters form very massive stars (VMSs) that can directly collapse to
intermediate-mass black holes (IMBHs). We follow the evolution of a pair of
neighbouring high-redshift mini-haloes with high-resolution, cosmological
hydrodynamical zoom-in simulations using the adaptive mesh refinement code
RAMSES combined with the non-equilibrium chemistry package KROME. The first
collapsing mini-halo is assumed to enrich the central nuclear star cluster
(NSC) of the other to a critical metallicity, sufficient for Population II
(Pop. II) star formation at redshift $z\approx27$. Using the spatial
configuration of the flattened, asymmetrical gas cloud forming in the core of
the metal enriched halo, we set the initial conditions for simulations of an
initially non-spherical star cluster with the direct summation code NBODY6
which are compared to about 2000 NBODY6 simulations of spherical star clusters
for a wide range of star cluster parameters. The final mass of the VMS that
forms depends strongly on the initial mass and initial central density of the
NSC. For the initial central densities suggested by our RAMSES simulations,
VMSs with mass > 400 M$_{\odot}$ can form in clusters with stellar masses of
$\approx10^4$ M$_{\odot}$, and this can increase to well over 1000 M$_{\odot}$
for more massive and denser clusters. The high probability we find for forming
a VMS in these mini-haloes at such an early cosmic time makes collisional
runaway of Pop. II star clusters a promising channel for producing large
numbers of high-redshift IMBHs that may act as the seeds of supermassive black
holes.
