The topology of cosmological reionization

Using the largest cosmological reionization simulation to date (similar to 24 billion particles), we use the genus curve to quantify the topology of the neutral hydrogen distribution on scales >= 1 Mpc h(-1) as it evolves during cosmological reionization. We find that the reionization process proceeds primarily in an inside-out fashion, in which higher density regions become ionized earlier than lower density regions. There are four distinct topological phases: ( 1) pre-reionization at z greater than or similar to 15, when the genus curve is consistent with a Gaussian density distribution; ( 2) preoverlap at 10 less than or similar to z less than or similar to 15, during which the number of H II bubbles increases gradually with time, until percolation of H II bubbles starts to take effect ( this phase is characterized by a very flat genus curve at high volume fractions); ( 3) overlap at 8 less than or similar to z less than or similar to 10, when large H II bubbles rapidly merge, manifested by a precipitous drop in the amplitude of the genus curve; and ( 4) postoverlap at 6 less than or similar to z less than or similar to 8, when H II bubbles have mostly overlapped, and the genus curve is consistent with a diminishing number of isolated neutral islands. After the end of reionization ( z less than or similar to 6), the genus of neutral hydrogen is consistent with Gaussian random phase, in agreement with observations.