PHEW: a parallel segmentation algorithm for three-dimensional AMR
  datasets - application to structure detection in self-gravitating flows
  We introduce PHEW (Parallel HiErarchical Watershed), a new segmentation
algorithm to detect structures in astrophysical fluid simulations, and its
implementation into the adaptive mesh refinement (AMR) code ramses. PHEW works
on the density field defined on the adaptive mesh, and can thus be used on the
gas density or the dark matter density after a projection of the particles onto
the grid. The algorithm is based on a "watershed" segmentation of the
computational volume into dense regions, followed by a merging of the segmented
patches based on the saddle point topology of the density field. PHEW is
capable of automatically detecting connected regions above the adopted density
threshold, as well as the entire set of substructures within. Our algorithm is
fully parallel and uses the MPI library. We describe in great detail the
parallel algorithm and perform a scaling experiment which proves the capability
of phew to run efficiently on massively parallel systems.
