Local reionization histories with a merger tree of the HII regions

¹ Kavli Institute for Cosmology and Institute of Astronomy, Madingley Rd, Cambridge CB3 0HA, UK
e-mail: jc@ast.cam.ac.uk
² Université de Strasbourg, CNRS UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France

Received: 24 July 2013
Accepted: 25 June 2014

Abstract

Aims. We investigate simple properties of the initial stage of the reionization process around progenitors of galaxies, such as the extent of the initial HII region before its fusion with the UV background, and the duration of its propagation.

Methods. We used a set of four reionization simulations with different resolutions and ionizing source prescriptions. By using a merger tree of the HII regions we compiled a catalog of the HII region properties. When the ionized regions undergo a major-merger event, we considered that they belong to the global UV background. From the lifetime of the region and from their volume until this moment we drew typical local reionization histories as a function of time and investigated the relation between these histories and the halo mass progenitors of the regions. We then used an average mass accretion history model (AMAH) to extrapolate the halo mass inside the region from high z to z = 0 to predict the past reionization histories of galaxies we see today.

Results. We found that the later an HII region appears during the reionization period, the shorter their related lifetime is and the smaller their volume before they merge with the global UV background. Quantitatively, the duration and extent of the initial growth of an HII region is strongly dependent on the mass of the inner halo and can be as long as ~50% of the reionization epoch. We found that the more massive a halo is today, the earlier it appears and the larger is the extension and the longer the propagation duration of its HII region. Quantitative predictions differ depending on the box size or the source model: small simulated volumes are affected by proximity effects between HII regions, and halo-based source models predict smaller regions and slower I-front expansion than models that use star particles as ionizing sources. Applying this extrapolation to Milky Way-type halos leads to a maximal extent of 1.1 Mpc/h for the initial HII region that established itself in ~150−200 ± 20 Myr. This is consistent with the prediction made using constrained Local Group simulations. For halos with masses similar to those of the Local Group (MW + M31), our result suggests that statistically it has not been influenced by an external front coming from a Virgo-like cluster.