Large-scale tidal fields on primordial density perturbations ? .II Alignment of Cosmic Structures

We show that the primordial density field imposes certain degree of coherence in the orientation of density perturbations. We quantify the scale of coherence and show that is significant on scales of at least $30-40h^{-1}$Mpc, being more important in density fields with flat spectrum. Evidence is also presented that the reason for this coherence is that the long density waves are the dominant part of the superposition of waves which build the large-scale density peaks. As a consequence of this, small-scale peaks tend to follow the configuration of their host large-scale perturbations. Different types of alignments are investigated. It is shown that alignment of the major axes of neighbouring peaks is more prominent around the highest peaks than around the lower ones. Alignments between the major axes of peaks and the radius-vector joining their centre with the centre of a high peak were not observed. Evidence is presented that peaks develop tails extending to neighbouring peaks, as predicted by Bond (1987a, 1987b) and Bahcall (1987).