Ionizing radiation in smoothed particle hydrodynamics

A new method for the inclusion of ionizing radiation from uniform radiation fields into 3D smoothed particle hydrodynamics (SPHI) simulations is prese nted. We calculate the optical depth for the Lyman continuum radiation from the source towards the SPHI particles by ray-tracing integration. The time -dependent ionization rate equation is then solved locally for the particle s within the ionizing radiation field. Using test calculations, we explore the numerical behaviour of the code with respect to the implementation of t he time-dependent ionization rate equation. We also test the coupling of th e heating caused by the ionization to the hydrodynamical part of the SPHI c ode.