A First Look at Galaxy Flyby Interactions: I. Characterizing the
  Frequency of Flybys in a Cosmological Context
  Hierarchical structure formation theory is based on the notion that mergers
drive galaxy evolution, so a considerable framework of semi-analytic models and
N-body simulations has been constructed to calculate how mergers transform a
growing galaxy. However, galaxy mergers are only one type of major dynamical
interaction between halos -- another class of encounter, a close flyby, has
been largely ignored. We use cosmological N-body simulations to reconstruct the
entire dynamical interaction history of dark matter halos. We present a careful
method of identifying and tracking a dark matter halo which resolves the
typical classes of anomalies that occur in N-body data. This technique allows
us to robustly follow halos and several hierarchical levels of subhalos as they
grow, dissolve, merge, and flyby one another -- thereby constructing both a
census of the dynamical interactions in a volume and an archive of the
dynamical evolution of an individual halo. In addition to a census of mergers,
our tool characterizes the frequency of close flyby interactions in the
Universe. We find that the number of close flyby interactions is comparable to,
or even surpasses, the number of mergers for halo masses $\gtrsim
10^{11}\,\Msun$ at $z \lesssim 2$. Halo flybys occur so frequently to high mass
halos that they are continually perturbed, unable to reach a dynamical
equilibrium. In particular, we find that Milky Way type halos undergo a similar
number of flybys as mergers irrespective of mass-ratio for $z\lesssim 2$. We
also find tentative evidence that at high redshift, $z \gtrsim 14$, flybys are
as frequent as mergers. Our results suggest that close halo flybys can play an
important role in the evolution of the earliest dark matter halos and their
galaxies, and can still influence galaxy evolution at the present epoch.
[Abridged]
