Observing Merger Trees in a New Light
  Merger trees harvested from cosmological $N$-body simulations encode the
assembly histories of dark matter halos over cosmic time, and are a fundamental
component of semi-analytical models (SAMs) of galaxy formation. The ability to
compare the tools used to construct merger trees, namely halo finders and tree
building algorithms, in an unbiased and systematic manner is critical to assess
the quality of merger trees. In this paper, we present the dendogram, a novel
method to visualise merger trees, which provides a comprehensive
characterisation of a halo's assembly history - tracking subhalo orbits, halo
merger events, and the general evolution of halo properties. We show the
usefulness of the dendogram as a diagnostic tool of merger trees by comparing
halo assembly histories from a single $N$-Body simulation analysed with three
different halo-finders -\textsc{VELOCIraptor}, \textsc{AHF} and
\textsc{Rockstar} - and their associated tree-builders. Based on our analysis
of the resulting dendograms, we highlight how they have been used to motivate
improvements to \textsc{VELOCIraptor}. The dendogram software is publicly
available online, at: https://github.com/rhyspoulton/MergerTree-Dendograms .
