Urchin: A Reverse Ray Tracer for Astrophysical Applications
  We describe URCHIN, a reverse ray tracing radiative transfer scheme optimised
to model self-shielding from the post-reionisation ultraviolet (UV) background
in cosmological simulations. The reverse ray tracing strategy provides several
benefits over forward ray tracing codes including: (1) the preservation of
adaptive density field resolution (2) completely uniform sampling of gas
elements by rays; (3) the preservation of galilean invariance; (4) the ability
to sample the UV background spectrum with hundreds of frequency bins; and (5)
exact preservation of the input UV background spectrum and amplitude in
optically thin gas. The implementation described here focuses on Smoothed
Particle Hydrodynamics (SPH). However, the method can be applied to any density
field representation in which resolution elements admit ray intersection tests
and can be associated with optical depths. We characterise the errors in our
implementation in stages beginning with comparison to known analytic solutions
and ending with a realistic model of the z = 3 cosmological UV background
incident onto a suite of spherically symmetric models of gaseous galactic
halos.
