Dark matter halo abundances, clustering and assembly histories at high redshift

We use a suite of high-resolution N-body simulations to study the properties, abundance and clustering of high-mass haloes at high redshift, including their mass assembly histories and mergers. We find that the analytic form which best fits the abundance of haloes depends sensitively on the assumed definition of halo mass, with common definitions of halo mass differing by a factor of 2 for these low-concentration, massive haloes. A significant number of massive haloes are undergoing rapid mass accretion, with major merger activity being common. We compare the mergers and mass accretion histories to the extended Press-Schechter formalism. We consider how major merger induced star formation or black hole accretion may change the distribution of photon production from collapsed haloes, and hence reionization, using some simplified examples. In all of these, the photon distribution for a halo of a given mass acquires a large scatter. If rare, high-mass haloes contribute significantly to the photon-production rates, the scatter in photon-production rate can translate into additional scatter in the sizes of ionized bubbles.