Alignments of Galaxy Group Shapes with Large Scale Structure

In this paper we analyse the alignment of galaxy groups with the surrounding large scale structure traced by spectroscopic galaxies from the Sloan Digital Sky Survey Data Release 7. We characterise these alignments by means of an extension of the classical two-point cross-correlation function, developed by Paz et al. 2008 (arXiv:0804.4477, MNRAS 389 1127). We find a strong alignment signal between the projected major axis of group shapes and the surrounding galaxy distribution up to scales of 30 Mpc/h. This observed anisotropy signal becomes larger as the galaxy group mass increases, in excellent agreement with the corresponding predicted alignment obtained from mock catalogues and LCDM cosmological simulations. These measurements provide new direct evidence of the adequacy of the gravitational instability picture to describe the large-scale structure formation of our Universe.