Research in Astron. Astrophys. 2009 Vol. 9 No. 1, 41 – 58 http://www.raa-journal.org http://www.iop.org/journals/raa Research in Astronomy and Astrophysics Alignment between galaxies and large-scale structure ∗ A. Faltenbacher 1,2 , Cheng Li 1,2 , Simon D. M. White 1 , Yi-Peng Jing 2 , Shu-De Mao 3 and Jie Wang 4 1 Max-Planck-Institute for Astrophysics, Karl-Schwarzschild-Str. 1, D-85741 Garching, Germany; afaltenbacher@mpa-garching.mpg.de 2 MPA/SHAO Joint Center for Astrophysical Cosmology at Shanghai Astronomical Observatory, Shanghai 200030, China 3 Jodrell Bank Centre for Astrophysics, Alan Turing Building, The University of Manchester, Manchester M13 9PL, UK 4 Department of Physics, Institute of Computational Cosmology, University of Durham, Science Laboratories, South Road, Durham DH1 3LE, UK Received 2008 November 12; accepted 2008 December 12 Abstract Based on the Sloan Digital Sky Survey DR6 (SDSS) and the Millennium Simulation (MS), we investigate the alignment between galaxies and large-scale struc- ture. For this purpose, we develop two new statistical tools, namely the alignment cor- relation function and the cos(2θ)-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. We apply the new statistics to the SDSS galaxy cat- alog. 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 h -1 Mpc. The signal increases with central galaxy luminosity. 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 that of the central region of the host halo (with size similar to that of the stellar galaxy). As an alternative, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of ∼ 25 ◦ . The misalignment decreases slightly with increasing luminosity of the central galaxy. Using the orientations and luminosities of the semi-analytic galaxies, we repeat our alignment analysis on mock surveys of the MS. Agreement with the SDSS results is good if the central orientations are used. Predictions using the halo orientations as proxies for cen- tral galaxy orientations overestimate the observed alignment by more than a factor of 2. Finally, the large volume of the MS allows us to generate a two-dimensional map of the alignment correlation function, which shows the reference galaxy distribution to be flat- tened 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 h -1 Mpc. Key words: dark matter halos: clustering — galaxies: large — scale structure of Universe — cosmology: theory — dark matter ∗ Supported by the National Natural Science Foundation of China.