Structure Finding in Cosmological Simulations: The State of Affairs
  The ever increasing size and complexity of data coming from simulations of
cosmic structure formation demands equally sophisticated tools for their
analysis. During the past decade, the art of object finding in these
simulations has hence developed into an important discipline itself. A
multitude of codes based upon a huge variety of methods and techniques have
been spawned yet the question remained as to whether or not they will provide
the same (physical) information about the structures of interest. Here we
summarize and extent previous work of the "halo finder comparison project": we
investigate in detail the (possible) origin of any deviations across finders.
To this extent we decipher and discuss differences in halo finding methods,
clearly separating them from the disparity in definitions of halo properties.
We observe that different codes not only find different numbers of objects
leading to a scatter of up to 20 per cent in the halo mass and Vmax function,
but also that the particulars of those objects that are identified by all
finders differ. The strength of the variation, however, depends on the property
studied, e.g. the scatter in position, bulk velocity, mass, and the peak value
of the rotation curve is practically below a few per cent, whereas derived
quantities such as spin and shape show larger deviations. Our study indicates
that the prime contribution to differences in halo properties across codes
stems from the distinct particle collection methods and -- to a minor extent --
the particular aspects of how the procedure for removing unbound particles is
implemented. We close with a discussion of the relevance and implications of
the scatter across different codes for other fields such as semi-analytical
galaxy formation models, gravitational lensing, and observables in general.
