THE DETECTION OF THE LARGE-SCALE ALIGNMENT OF MASSIVE GALAXIES AT z ∼ 0.6

We report on the detection of the alignment between galaxies and large-scale structure at z ~ 0.6 based on the CMASS galaxy sample from the Baryon Oscillation Spectroscopy Survey Data Release 9. We use two statistics to quantify the alignment signal: (1) the alignment two-point correlation function that probes the dependence of galaxy clustering at a given separation in redshift space on the projected angle (? p ) between the orientation of galaxies and the line connecting to other galaxies, and (2) the cos (2?)-statistic that estimates the average of cos (2? p ) for all correlated pairs at a given separation s. We find a significant alignment signal out to about 70 h ?1?Mpc in both statistics. Applications of the same statistics to dark matter halos of mass above 1012 h ?1 M ? in a large cosmological simulation show scale-dependent alignment signals similar to the observation, but with higher amplitudes at all scales probed. We show that this discrepancy may be partially explained by a misalignment angle between central galaxies and their host halos, though detailed modeling is needed in order to better understand the link between the orientations of galaxies and host halos. In addition, we find systematic trends of the alignment statistics with the stellar mass of the CMASS galaxies, in the sense that more massive galaxies are more strongly aligned with the large-scale structure.