RAY-RAMSES: a code for ray tracing on the fly in N-body simulations
  We present a ray tracing code to compute integrated cosmological observables
on the fly in AMR N-body simulations. Unlike conventional ray tracing
techniques, our code takes full advantage of the time and spatial resolution
attained by the N-body simulation by computing the integrals along the line of
sight on a cell-by-cell basis through the AMR simulation grid. Moroever, since
it runs on the fly in the N-body run, our code can produce maps of the desired
observables without storing large (or any) amounts of data for post-processing.
We implemented our routines in the RAMSES N-body code and tested the
implementation using an example of weak lensing simulation. We analyse basic
statistics of lensing convergence maps and find good agreement with
semi-analytical methods. The ray tracing methodology presented here can be used
in several cosmological analysis such as Sunyaev-Zel'dovich and integrated
Sachs-Wolfe effect studies as well as modified gravity. Our code can also be
used in cross-checks of the more conventional methods, which can be important
in tests of theory systematics in preparation for upcoming large scale
structure surveys.
