The Distribution of Satellite Galaxies in a Λcdm Universe

We investigate the distribution of luminous satellite galaxies with respect to the symmetry axes of isolated host galaxies in a ΛCDM universe. The selection of “host” and “satellite” samples based upon three somewhat different algorithms, all of which yield nearly identical results. In projection on the sky, the satellites are located preferentially close to the major axes of the host halos, resulting in a substantially flattened distribution of satellites. The flatness of the satellite distribution compares remarkably well with the flatness of the dark mass associated with the host galaxies. The degree to which the satellites are distributed anisotropically with respect to the major axes of the projected host halos is, however, grossly in excess of the degree to which satellites in the Sloan Digital Sky Survey (SDSS) are distributed anisotropically with respect to the major axes of the images of the host galaxies. This may be an indication that the mass and light of the SDSS hosts is not in perfect alignment on the sky. To investigate this we use a number of prescriptions in which the angular momentum vectors of the host galaxies are closely aligned with the principle axes of the surrounding dark mass in order to assign position angles to images of the hosts. When the distribution of the satellites is then computed with respect to the major axes of the images of the hosts, the anisotropy in the distribution of satellite galaxies is reduced significantly In particular, when the angular momentum vectors of the hosts are aligned with the minor axes of the dark mass contained within radii of r200 or r100, the anisotropy is reduced to a level that is comparable to the anisotropy shown by the SDSS satellites. The reduction in the anisotropy is due to a combination of the misalignment of mass and light for the host galaxies and the fact that the ellipticity of the image of a host galaxy is a rather poor indicator of the ellipticity of its projected halo. Lastly, the anisotropy in the distribution of the satellites persists to projected radii rp ∼ 500 kpc and likely reflects preferential mass accretion along filaments. Subject headings: dark matter — galaxies: fundamental parameters — galaxies: dwarf — galaxies: halos — galaxies: structure