  We use two cosmological simulations of structure formation to study the
conditions under which dark matter haloes emerge from the linear density field.
Our analysis focuses on matching sites of halo collapse to local density
maxima, or "peaks", in the initial conditions of the simulations and provides a
crucial test of the central ansatz of the peaks formalism. By identifying peaks
on a variety of smoothed, linearly extrapolated density fields we demonstrate
that as many as ~70% of well-resolved dark matter haloes form preferentially
near peaks whose characteristic masses are similar to that of the halo, with
more massive haloes showing a stronger tendency to reside near peaks initially.
We identify a small but significant fraction of haloes that appear to evolve
from peaks of substantially lower mass than that of the halo itself. We refer
to these as "peakless haloes" for convenience. By contrasting directly the
properties of these objects with the bulk of the proto-halo population we find
two clear differences: 1) their initial shapes are significantly flatter and
more elongated than the predominantly triaxial majority, and 2) they are, on
average, more strongly compressed by tidal forces associated with their
surrounding large scale structure. Using the two-point correlation function we
show that peakless haloes tend to emerge from highly clustered regions of the
initial density field implying that, at fixed mass, the accretion geometry and
mass accretion histories of haloes in highly clustered environments differ
significantly from those in the field. This may have important implications for
understanding the origin of the halo assembly bias, of galaxy properties in
dense environments and how environment affects the morphological transformation
of galaxies near groups and rich galaxy clusters.

Cosmology and Nongalactic Astrophysics