RADAMESH: Cosmological Radiative Transfer for Adaptive Mesh Refinement Simulations

TL;DR

RADAMESH is a new adaptive, ray-tracing radiative transfer code that efficiently models ionization fronts in cosmological simulations, improving accuracy and speed through innovative Monte Carlo sampling and mesh refinement.

Contribution

The paper introduces RADAMESH, a novel RT code with adaptive mesh refinement and a cell-by-cell Monte Carlo approach, enhancing resolution and computational efficiency in cosmological ionization modeling.

Findings

Accurately resolves ionization fronts in large cosmological boxes.

Significantly increases temperature predictions within HII regions.

Recombination radiation notably affects ionization states compared to on-the-spot approximation.

Abstract

We present a new three-dimensional radiative transfer (RT) code, RADAMESH, based on a ray-tracing, photon-conserving and adaptive (in space and time) scheme. RADAMESH uses a novel Monte Carlo approach to sample the radiation field within the computational domain on a "cell-by-cell" basis. Thanks to this algorithm, the computational efforts are now focused where actually needed, i.e. within the Ionization-fronts (I-fronts). This results in an increased accuracy level and, at the same time, a huge gain in computational speed with respect to a "classical" Monte Carlo RT, especially when combined with an Adaptive Mesh Refinement (AMR) scheme. Among several new features, RADAMESH is able to adaptively refine the computational mesh in correspondence of the I-fronts, allowing to fully resolve them within large, cosmological boxes. We follow the propagation of ionizing radiation from an arbitrary number of sources and from the recombination radiation produced by H and He. The chemical state of six species (HI, HII, HeI, HeII, HeIII, e) and gas temperatures are computed with a time-dependent, non-equilibrium chemistry solver. We present several validating tests of the code, including the standard tests from the RT Code Comparison Project and a new set of tests aimed at substantiating the new characteristics of RADAMESH. Using our AMR scheme, we show that properly resolving the I-front of a bright quasar during Reionization produces a large increase of the predicted gas temperature within the whole HII region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Stroemgren sphere test) with respect to the widely used "on-the-spot" approximation.