A study of simulated histories of reionization with merger trees of HII
  regions
  We describe a new methodology to analyze the reionization process in
numerical simulations: the chronology and the geometry of reionization is
investigated by means of merger histories of individual HII regions. From the
merger tree of ionized patches, one can track the individual evolution of the
regions properties such as e.g. their size, or the intensity of the percolation
process by looking at the formation rate, the frequency of mergers and the
number of individual HII regions involved in the mergers. We apply the merger
tree technique to simulations of reionization with three different kinds of
ionizing source models and two resolutions. Two of them use star particles as
ionizing sources. In this case we confront two emissivity evolutions for the
sources in order to reach the reionization at z ~ 6. As an alternative we built
a semi-analytical model where the dark matter halos extracted from the density
fields are assumed as ionizing sources. We then show how this methodology is a
good candidate to quantify the impact of the adopted star formation on the
history of the observed reionization. The semi-analytical model shows a
homogeneous reionization history with 'local' hierarchical growth steps for
individual HII regions. On the other hand auto-consistent models for star
formation tend to present fewer regions with a dominant region in size which
governs the fusion process early in the reionization at the expense of the
'local' reionizations. The differences are attenuated when the resolution of
the simulation is increased.
