Galaxy–group (halo) alignments from SDSS DR7 and the ELUCID simulation

Abstract

ABSTRACT Based on galaxies from the Sloan Digital Sky Survey (SDSS) and subhaloes in the corresponding reconstructed region from the constrained simulation of ELUCID, we study the alignment of central galaxies relative to their host groups in the group catalogue, as well as the alignment relative to the corresponding subhaloes in the ELUCID simulation. Galaxies in observation are matched to dark matter subhaloes in the ELUCID simulation using a novel neighborhood abundance matching method. In observation, the major axes of galaxies are found to be preferentially aligned to the major axes of their host groups. There is a colour dependence of galaxy–group alignment that red centrals have a stronger alignment along the major axes of their host groups than blue centrals. Combining galaxies in observation and subhaloes in the ELUCID simulation, we also find that central galaxies have their major axes to be aligned to the major axes of their corresponding subhaloes in the ELUCID simulation. We find that the galaxy–group and galaxy–subhalo alignment signals are stronger for galaxies in more massive haloes. We find that the alignments between main subhaloes and the SDSS-matched subhalo systems in simulation are slightly stronger than the galaxy–group alignments in observation.