The Topology of Reionization

Using the largest cosmological reionization simulation to-date (∼24 billion particles) , we use the genus curve to quantify the topology of the neutral hydrogen distribution on scales ≥ 1Mpch as it evolves during cosmological reionization. We find that the reionization process proceeds primarily in an inside-out fashion, where higher density regions become ionized earlier than lower density regions. There are four distinct topological phases: (1) Pre-reionization at z & 15, when the genus curve is consistent with a Gaussian density distribution. (2) Pre-overlap at 10 . z . 15, during which the number of HII bubbles increases gradually with time, until percolation of HII bubbles starts to take effect, characterized by a very flat genus curve at high volume fractions. (3) Overlap at 8 . z . 10, when large HII bubbles rapidly merge, manifested by a precipitous drop in the amplitude of the genus curve. (4) Post-overlap at 6 . z . 8, when HII bubbles have mostly overlapped and the genus curve is consistent with a diminishing number of isolated neutral islands. After the end of reionization (z . 6), the genus of neutral hydrogen is consistent with Gaussian random phase, in agreement with observations. Subject headings: cosmology: theory — intergalactic medium — large-scale structure of universe — methods: numerical —methods: statistical — radiative transfer