A study of simulated reionization histories with merger trees of HII regions

Aims. We describe a new methodology for analyzing the reionization process in numerical simulations. The chronology and the geometry of reionization is investigated by the merger histories of individual HII regions. Methods. From the merger tree of ionized patches, one can track the individual evolution of the regions properties, such as their size, 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 applied 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 compared two emissivity evolutions for the sources to reach the reionization at z similar to 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. Results. 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, autoconsistent models for star formation tend to present fewer regions, with a dominant region in size that 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.