On the accuracy of common momentbased radiative transfer methods for simulating reionization
Abstract
Modern cosmological simulations of reionization often treat the radiative transfer by solving for the monopole and dipoles of the intensity field and by making some ansatz for the quadrupole moments to close the system of equations. We investigate the accuracy of the most common closure methods, i.e. Eddington tensor choices. We argue that these algorithms are the most likely to err after reionization and study qausianalytic test problems that mimic these situations: largescale fluctuations in the postreionization ionizing background and radiative transfer in a predominantly ionized medium with discrete absorbers. We show that the usual closure methods, OTVET and M1, overionize selfshielding absorbers when fixing the background photoionization rate, leading to 3040% higher emissivity to balance the increased recombination rate. This overionization results in a simulation run with these algorithms having a factor of ~ 2 lower average metagalactic photoionization rate relative to truth for a given ionizing emissivity. Furthermore, these algorithms are unlikely to reproduce fluctuations in the ionizing background on scales below the photon mean path: OTVET tends to overpredict the fluctuations there when the simulation box is smaller than twice the mean free path and underpredict otherwise, while M1 drastically underpredicts these fluctuations. As a result, these numerical methods are likely not sufficiently accurate to interpret the Lyα forest opacity fluctuations observed after reionzation. We also comment on ray tracing methods, showing that a high number of angular directions need to be followed to capture fluctuations in the postreionization ionizing background accurately. Lastly, we argue that the strong dependence of the postreionization ionizing background on the value of the reduced speed of light found in many simulations signals that the ionizing photon mean free path is several times larger in such simulations than the observationally measured value.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 February 2021
 DOI:
 10.1088/14757516/2021/02/042
 arXiv:
 arXiv:2009.07278
 Bibcode:
 2021JCAP...02..042W
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 revision 1, submitted to JCAP