Alignment between galaxies and large-scale structure

Abstract

Based on the Sloan Digital Sky Survey DR6 (SDSS) and Millennium Simulation (MS) we investigate the alignment between galaxies and large-scale structure. For this purpose we develop two new statistical tools, namely the alignment correlation function and the cos(2theta)-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. Applied to the SDSS galaxy catalog the alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous (L > L*) galaxies out to projected separations of 60 Mpc/h. No alignment signal is detected for blue galaxies. The cos(2θ)-statistic yields very similar results. Starting from a MS semi-analytic galaxy catalog we assign an orientation to each red, luminous and central galaxy, based on the central region of the host halo. Alternatively, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of ~25 deg. Agreement with the SDSS results is good if the central orientations are used. Using the halo orientations overestimates the observed alignment by more than a factor of 2. The large volume of the MS allows to generate two-dimensional maps of the alignment correlation function which show the reference galaxy distribution to be flattened parallel to the orientations of red luminous galaxies with axis ratios of ~0.5 and ~0.75 for halo and central orientations,respectively. These ratios are almost independent of scale out to 60 Mpc/h.