Dark-ages Reionization & Galaxy Formation Simulation I: The dynamical
  lives of high redshift galaxies
  We present the Dark-ages Reionization and Galaxy-formation Observables from
Numerical Simulations (DRAGONS) program and Tiamat, the collisionless N-body
simulation program upon which DRAGONS is built. The primary trait
distinguishing Tiamat from other large simulation programs is its density of
outputs at high redshift (100 from z=35 to z=5; roughly one every 10 Myr)
enabling the construction of very accurate merger trees at an epoch when galaxy
formation is rapid and mergers extremely frequent. We find that the
friends-of-friends halo mass function agrees well with the prediction of Watson
et al. at high masses, but deviates at low masses, perhaps due to our use of a
different halo finder or perhaps indicating a break from "universal" behaviour.
We then analyse the dynamical evolution of galaxies during the Epoch of
Reionization finding that only a small fraction (~20%) of galactic halos are
relaxed. We illustrate this using standard relaxation metrics to establish two
dynamical recovery time-scales: i) halos need ~1.5 dynamical times following
formation, and ii) ~2 dynamical times following a major (3:1) or minor (10:1)
merger to be relaxed. This is remarkably consistent across a wide mass range.
Lastly, we use a phase-space halo finder to illustrate that major mergers drive
long-lived massive phase-space structures which take many dynamical times to
dissipate. This can yield significant differences in the inferred mass build-up
of galactic halos and we suggest that care must be taken to ensure a physically
meaningful match between the galaxy-formation physics of semi-analytic models
and the halo finders supplying their input.
