Galaxy-group (halo) alignments from SDSS DR7 and the ELUCID simulation
  Based on galaxies from the Sloan Digital Sky Survey (SDSS) and subhalos 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 catalog, as well as the alignment relative to the
corresponding subhalos in the ELUCID simulation. Galaxies in observation are
matched to dark matter subhalos 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 color 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 subhalos in the ELUCID
simulation, we also find that central galaxies have their major axes to be
aligned to the major axes of their corresponding subhalos in the ELUCID
simulation. We find that the galaxy-group and galaxy-subhalo alignment signals
are stronger for galaxies in more massive halos. We find that the alignments
between main subhalos and the SDSS matched subhalo systems in simulation are
slightly stronger than the galaxy-group alignments in observation.
