Alignment between luminous red galaxies and surrounding structures at z ∼ 0.5

Abstract

We analyse a high-redshift sample (0.4 < z < 0.5) of luminous red galaxies (LRGs) extracted from the Sloan Digital Sky Survey data release 4 and their surrounding structures to explore the presence of alignment effects of these bright galaxies with neighbour objects. In order to avoid projection effects, we compute photometric redshifts for galaxies within 3 h-1 Mpc in projection of LRGs and calculate the relative angle between the LRG major axis and the direction to neighbours within 1000 km s-1. We find a clear signal of alignment between LRG orientations and the distribution of galaxies within 1.5 h-1 Mpc. The alignment effects are present only for the red population of tracers; LRG orientation is uncorrelated to the blue population of neighbour galaxies. These results add evidence to the alignment effects between primaries and satellites detected at low redshifts. We conclude that such alignments were already present at z ∼ 0.5. © 2006 RAS.