MEGA: Merger graphs of structure formation
  When following the growth of structure in the Universe, we propose replacing
merger trees with merger graphs, in which haloes can both merge and split into
separate pieces. We show that this leads to smoother mass growth and eliminates
catastrophic failures in which massive haloes have no progenitors or
descendants. For those who prefer to stick with merger trees, we find that
trees derived from our merger graphs have similar mass growth properties to
previous methods, but again without catastrophic failures. For future galaxy
formation modelling, two different density thresholds can be used to
distinguish host haloes (extended galactic haloes, groups and clusters) from
higher-density subhaloes: sites of galaxy formation.
