The shape and size distribution of H II regions near the percolation transition
Abstract
Using Shapefinders, which are ratios of Minkowski functionals, we study the morphology of neutral hydrogen (H I) density fields, simulated using seminumerical technique (insideout), at various stages of reionization. Accompanying the Shapefinders, we also employ the `largest cluster statistic' (LCS), originally proposed in Klypin & Shandarin, to study the percolation in both neutral and ionized hydrogen. We find that the largest ionized region is percolating below the neutral fraction x_{H I}≲ 0.728 (or equivalently z ≲ 9). The study of Shapefinders reveals that the largest ionized region starts to become highly filamentary with nontrivial topology near the percolation transition. During the percolation transition, the first two Shapefinders  `thickness' (T) and `breadth' (B)  of the largest ionized region do not vary much, while the third Shapefinder  `length' (L)  abruptly increases. Consequently, the largest ionized region tends to be highly filamentary and topologically quite complex. The product of the first two Shapefinders, T × B, provides a measure of the `crosssection' of a filamentlike ionized region. We find that, near percolation, the value of T × B for the largest ionized region remains stable at ∼7 Mpc^{2} (in comoving scale) while its length increases with time. Interestingly, all large ionized regions have similar crosssections. However, their length shows a powerlaw dependence on their volume, L ∝ V^{0.72}, at the onset of percolation.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 June 2018
 DOI:
 10.1093/mnras/sty714
 arXiv:
 arXiv:1801.01116
 Bibcode:
 2018MNRAS.477.1984B
 Keywords:

 intergalactic medium;
 dark ages;
 reionization;
 first stars;
 largescale structure of Universe;
 cosmology: theory;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 10 pages, 9 figures, matches published version in MNRAS