radamesh: cosmological radiative transfer for Adaptive Mesh Refinement simulations

We present a new three-dimensional radiative transfer (RT) code, radamesh (Radiative-transfer on ADAptive MESH), 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 (H i, H ii, He i, He ii, He iii, 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 H ii region. Also, we discuss how H and He recombination radiation is able to substantially change the ionization state of both species (for the classical Strömgren sphere test) with respect to the widely used ‘on-the-spot’ approximation.