“New radiative transfer methods in numerical simulation of the epoch of reionisation“
In current cosmological simulations, the radiative transfer modules generally rely on the M1 approximation, which has some glaring flaws related to its fluid-like behaviour, such as spurious pseudo-sources and loss of directionality when radiation fronts from different directions collide. PN, another moment-based model used in other fields of physics, may correct these issues. We aim at testing out PN in an astrophysical setting and compare it to M1, in order to see if it can indeed correct M1’s flaws. Also, we want to use PN’s solutions to better pinpoint M1 errors. We implement a PN radiation transport method and couple it to a photo-thermo-chemistry module to account for the interaction of ionising radiation with the Hydrogen gas, and benchmark it using tests for “radiative transfer models comparison in astrophysics” as defined in Iliev et al (2006). We find that high orders PN (e.g. P9) indeed correct M1’s flaws, while faring as well or even better in some aspects in the tests, in particular when directionality is important or colliding radiation fronts occur. By comparing P9 and M1 radiation fields in an idealised and cosmological test case, we highlight a new, thus far unreported artefact of M1, the ‘dark sombrero’. A dark sombrero appears as a spherical photon-deficit shell around the source. The photon density in dark sombreros can be underestimated by a factor up to 2-3. They occur in regions where a source’s radiation field connects with that of another source or group of sources. These basic properties (position and amplitude) of the dark sombreros may depend on the sources’ relative intensities, positions, spatial resolution, although we have not been able to test this in detail in this work. Moreover, the M1 larger scale photon density also exhibits spurious features, enhancing or reducing photon density in various regions. We use a small reionisation-like test simulation to characterise the relative error in hydrogen neutral fractions between M1 and P9. The relative error is well represented by a Gaussian with a dispersion of 0.27 dex in log10(xHI). Both aspects are likely related to the photons’ collisional behaviour in M1.