Artist: fast radiative transfer for large-scale simulations of the epoch of reionization

Abstract

We introduce the "Asymmetric Radiative Transfer In Shells Technique" (Artist), a new method for photon propagation on large scales that explicitly conserves photons, propagates photons at the speed of light, approximately accounts for photon directionality, and closely reproduces results of more detailed radiative transfer (RT) codes. Crucially, it is computationally fast enough to evolve the large cosmological volumes required to predict the 21cm power spectrum on scales that will be probed by future experiments targeting the Epoch of Reionisation (EoR). Most semi-numerical models aimed at predicting the EoR 21cm signal make use of an excursion set formalism (ESF) approach, which achieves computational viability by compromising on photon conservation, constraining ionised regions to be spherical by construction, and not accounting for light-travel time. By implementing our RT method within the semi-numerical code SimFast21, we show that Artist predicts a significantly different evolution for the EoR ionisation field compared to the code's native ESF. In particular, Artist predicts a more gradual evolution of the volume-averaged ionisation fraction, and up to an order-of-magnitude difference in the ionisation power, depending on the physical parameters assumed. Its application to large-scale EoR simulations will therefore allow more physically-motivated constraints to be obtained for key EoR parameters, such as the escape fraction. and moment-based methods, with different strengths and weaknesses, balancing accuracy versus computational efficiency (Trac & Gnedin 2011). Often, while these methods can be made optimally accurate with sufficient computational investment, they remain computationally prohibitive in large-scale cosmological simulations that seek to reproduce the evolution of the universe on at least tens of Mpc scales, while simultaneously ensuring that the injection and propagation of photons on the smallest scales is both accurate and self-consistent.One particular such case, currently at the forefront of astrophysical research, is the modelling of the last global phase-change in the history of the universe -the epoch of reionisation (EoR). The sources of the photons respon-

Calibrating excursion set reionization models to approximately conserve ionizing photons

“…Therefore, without accounting for this non-conservation when using semi-numerical EoR simulations to infer galaxy properties from observations, we expect a bias in the recovered ionizing emissivity (e.g. an overestimate of the ionizing photon escape fraction; Hutter 2018; Molaro et al 2019).…”

“…Several approaches have been developed to conserve ionizing photons in approximate radiative transfer schemes (e.g. Paranjape et al 2016;Kim et al 2016;Hassan et al 2017;Choudhury & Paranjape 2018;Molaro et al 2019). However, these can come at the cost of significantly increased computation time and/or stepping away from the well-tested excursion set framework entirely.…”

The effect of inhomogeneous reionisation on the Lyman-$α$ forest power spectrum at redshift $z>4$: implications for thermal parameter recovery

“…One of the most expensive physical processes to implement is the prescription for the radiative transfer of UV photons during inhomogeneous reionisation (Gnedin 2000;Razoumov et al 2002;Ciardi et al 2003;Mellema et al 2006;Finlator et al 2018;Molaro et al 2019). A common approximation used in hydrodynamical simulations of the Ly𝛼 forest that side-stpdf this requirement are precomputed, spatially homogeneous ultraviolet background (UVB) models (Haardt & Madau 2012;Puchwein et al 2019;Faucher-Giguère 2020).…”

The subtlety of Ly-a photons: changing the expected range of the 21-cm signal

“…The well-known WF effect, in which the absorption and re-emission of ambient Ly-α photons by the hydrogen atoms drives their spin temperature towards the kinetic gas temperature, renders the 21-cm line visible against the background radiation (Madau et al 1997;Barkana 2018;Mesinger 2019). Stellar Lyman photons redshifted to the vicinity of the Ly-α line experience multiple scattering (Semelin et al 2007;Naoz & Barkana 2008;Baek et al 2009;Visbal & McQuinn 2018;Molaro et al 2019;Reis et al 2020a) which affects their spatial distribution, and, thus, affect the resulting 21-cm signal. The effect of multiple scattering is rarely fully included in the 21-cm simulations.…”

“…In this work we explore the impact on the global 21-cm signal and fluctuations of the subtle Ly-α effects that are often neglected, namely (i) the effect of multiple scattering of Ly-α photons on the spatial distribution of these photons and, thus, the WF coupling (Semelin et al 2007;Naoz & Barkana 2008;Baek et al 2009;Visbal & McQuinn 2018;Molaro et al 2019;Reis et al 2020a), (ii) the contributions to the IGM heating of Ly-α photons (Madau et al 1997) and (iii) heating of the IGM by the CMB photons (Venumadhav et al 2018). We assess the importance of these contributions over the vast allowed astrophysical landscape.…”