To carry out this calculation we added some useful new technical features to Galacticus. It's always been possible to attach "events" to halos in a Galacticus merger tree - when the halo reaches the event, some action is triggered before the halo is allowed to continue evolving. Now, it's possible to attach events to the entire universe of merger trees in Galacticus (and also to individual trees, but that's another story). Such a "universe event" causes all merger trees to halt at the event time. In the case of this background radiation calculation, universe events are used to halt the evolution of merger trees, compute their net emissivity, evolve the background radiation, and then let the trees evolve further until they reach the next universe event. Galacticus handles all of this transparently - it normally tries to evolve one tree at a time and then destroy it in order to keep its memory footprint small, but if a universe event is added it will process all merger trees simultaneously. This uses more memory, but allows for calculations that depend on the properties of the entire population of galaxies across all merger trees.
Look for more results utilizing this background radiation calculation later this year!