Galaxies in a Λ CDM Universe

We compute the locations of satellite galaxies with respect to their hosts using the ΛCDM GIF simulation. If the major axes of the hosts' images are perfectly aligned with the major axes of their projected mass, the satellites are located preferentially close to the hosts' major axes. In this case, the degree of anisotropy in the satellite locations is a good tracer of the flattening of the hosts' halos. If all hosts have luminous circular disks, the symmetry axes of the projected mass and light are not perfectly aligned, and the locations of the satellites depend upon how the hosts' disks are placed within their halos. If the disk angular momentum vectors are aligned with the major axes of the halos, the satellites show a pronounced " Holmberg effect ". If the disk angular momentum vectors are aligned with the intermediate axes of the local large scale structure, the distribution of satellite locations is essentially isotropic. If the disk angular momentum vectors are aligned with either the minor axes or with the net angular momentum vectors of the halos, the satellites are distributied anisotropically about their hosts, with a preference for being found nearby the hosts' major axes. This agrees well with the observation that satellite galaxies in the Sloan Digital Sky Survey tend to be found nearby the major axes of their hosts, and suggests that the mass and light of SDSS host galaxies must be fairly well aligned in projection on the sky.