A study of relative velocity statistics in Lagrangian perturbation
  theory with PINOCCHIO
  Subject of this paper is a detailed analysis of the PINOCCHIO algorithm for
studying the relative velocity statistics of merging haloes in Lagrangian
perturbation theory. Given a cosmological background model, a power spectrum of
fluctuations as well as a Gaussian linear density contrast field $\delta_{\rm
l}$ is generated on a cubic grid, which is then smoothed repeatedly with
Gaussian filters. For each Lagrangian particle at position $\bmath{q}$ and each
smoothing radius $R$, the collapse time, the velocities and ellipsoidal
truncation are computed using Lagrangian Perturbation Theory. The collapsed
medium is then fragmented into isolated objects by an algorithm designed to
mimic the accretion and merger events of hierarchical collapse. Directly after
the fragmentation process the mass function, merger histories of haloes and the
statistics of the relative velocities at merging are evaluated. We
reimplemented the algorithm in C++, recovered the mass function and optimised
the construction of halo merging histories. Comparing our results with the
output of the Millennium simulation suggests that PINOCCHIO is well suited for
studying relative velocities of merging haloes and is able to reproduce the
pairwise velocity distribution.
