Photon number conservation and the largescale 21 cm power spectrum in seminumerical models of reionization
Abstract
Seminumerical models of the reionization of neutral hydrogen (HI) based on the excursion set (ES) approach are known to violate photon number conservation at the few per cent level. In this work, we highlight a more severe, previously unrecognized shortcoming of ES models: the largescale 21 cm power spectrum (equivalently, H I bias b_{H I}) is a relatively strong function of the spatial resolution used to generate ES ionization maps. We trace this problem to the fact that photon nonconservation in these models arises from a resolutiondependent mixture of spatially resolved, photon nonconserving bubbles, and partially ionized grid cells which are perfectly photon conserving (PC) by construction. We argue that this inevitably leads to a resolution dependence of b_{H I}, with the correct, converged value only emerging at very coarse resolution. Quantitatively, we find that b_{H I} can be nonconverged by as much as ∼2025{{ per cent}} in conservative ES implementations with grid sizes Δx = 510h^{1}cMpc, even when photon nonconservation is as small as ∼34{{ per cent}}. Thus, although numerically efficient, ES ionization maps coarse enough to produce a converged H I bias would wash out all topological features of the ionization field at scales k≳ 0.05 h{^{1}}cMpc. We therefore present a new, explicitlyPC seminumerical algorithm which distributes photons isotropically around sources while also accounting for anisotropic overlaps between nearby bubbles. Our PC algorithm predicts a resolutionindependent value of b_{H I} consistent with the result of lowresolution ES maps, thus serving as a useful compromise between standard ES implementations and more expensive radiative transfer simulations.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 December 2018
 DOI:
 10.1093/mnras/sty2551
 arXiv:
 arXiv:1807.00836
 Bibcode:
 2018MNRAS.481.3821C
 Keywords:

 intergalactic medium;
 cosmology: theory;
 dark ages;
 reionization;
 first stars;
 largescale structure of Universe;
 Astrophysics  Cosmology and Nongalactic Astrophysics
 EPrint:
 Accepted for publication in MNRAS