Extracting galaxy merger timescales I: Tracking haloes with WhereWolf
  and spinning orbits with OrbWeaver
  Hierarchical models of structure formation predict that dark matter halo
assembly histories are characterised by episodic mergers and interactions with
other haloes. An accurate description of this process will provide insights
into the dynamical evolution of haloes and the galaxies that reside in them.
Using large cosmological N-body simulations, we characterise halo orbits to
study the interactions between substructure haloes and their hosts, and how
different evolutionary histories map to different classes of orbits. We use two
new software tools - WhereWolf, which uses halo group catalogues and merger
trees to ensure that haloes are tracked accurately in dense environments, and
OrbWeaver, which quantifies each halo's orbital parameters. We demonstrate how
WhereWolf improves the accuracy of halo merger trees, and we use OrbWeaver to
quantify orbits of haloes. We assess how well analytical prescriptions for the
merger timescale from the literature compare to measured merger timescales from
our simulations and find that existing prescriptions perform well, provided the
ratio of substructure-to-host mass is not too small. In the limit of small
substructure-to-host mass ratio, we find that the prescriptions can
overestimate the merger timescales substantially, such that haloes are
predicted to survive well beyond the end of the simulation. This work
highlights the need for a revised analytical prescription for the merger
timescale that more accurately accounts for processes such as catastrophic
tidal disruption.
