标题： 一种用于宇宙学流体动力学模拟中精确辐射转移的新方法 显示英文标题

标题： A novel approach for accurate radiative transfer in cosmological hydrodynamic simulations

摘要： 我们提出了一种基于显式光子守恒对流方案的辐射转移数值实现，其中结构化或非结构化网格单元界面的辐射通量是通过强度场的二阶重构来计算的。 该方法采用玻尔兹曼形式的辐射转移方程的直接离散化，具有可调的角分辨率，在原则上在光学薄和光学厚区域都能同样有效。 在我们对该方案最一般的表述中，局部辐射场被分解为等立体角方向区间的线性求和，这些区间铺满单位球面。 每个这样的“锥形场”独立传输，在锥形内的方向上强度保持恒定。 光子以光速传播（或可选地使用降低的光速近似以允许更大的时间步长），从而得到一个完全时间依赖的辐射转移方程解，能够自然地处理任意数量的光源以及散射。 该方法可以产生清晰的阴影，但受所采用的角分辨率的限制。 如果点光源数量较少且散射不重要，我们的实现可以以角空间精确处理每个光源，产生的阴影清晰度仅受限于网格分辨率。 第三种混合方法是仅对局部最明亮的少量点光源进行显式处理，其余辐射强度则用辐射扩散近似来追踪。 我们已在移动网格代码{\small AREPO}中实现了该方法，其中采用操作符分裂方法将辐射转移与流体力学耦合，交替地对辐射转移进行子循环和流体力学演化步骤。 显示英文摘要

摘要： We present a numerical implementation of radiative transfer based on an explicitly photon-conserving advection scheme, where radiative fluxes over the cell interfaces of a structured or unstructured mesh are calculated with a second-order reconstruction of the intensity field. The approach employs a direct discretisation of the radiative transfer equation in Boltzmann form with adjustable angular resolution that in principle works equally well in the optically thin and optically thick regimes. In our most general formulation of the scheme, the local radiation field is decomposed into a linear sum of directional bins of equal solid-angle, tessellating the unit sphere. Each of these "cone-fields" is transported independently, with constant intensity as a function of direction within the cone. Photons propagate at the speed of light (or optionally using a reduced speed of light approximation to allow larger timesteps), yielding a fully time-dependent solution of the radiative transfer equation that can naturally cope with an arbitrary number of sources, as well as with scattering. The method casts sharp shadows, subject to the limitations induced by the adopted angular resolution. If the number of point sources is small and scattering is unimportant, our implementation can alternatively treat each source exactly in angular space, producing shadows whose sharpness is only limited by the grid resolution. A third hybrid alternative is to treat only a small number of the locally most luminous point sources explicitly, with the rest of the radiation intensity followed in a radiative diffusion approximation. We have implemented the method in the moving-mesh code {\small AREPO}, where it is coupled to the hydrodynamics in an operator splitting approach that subcycles the radiative transfer alternatingly with the hydrodynamical evolution steps.