Lopsided Satellite Distributions around Isolated Host Galaxies

We investigate the spatial distribution of the satellites surrounding 789 isolated host galaxies in the NASA-Sloan Atlas. The host-satellite systems are identified using typical redshift space selection criteria and, unlike similar previous studies, all systems in our sample have at least five satellites. On average, the satellites of our red hosts are found preferentially close to their hosts’ major axes, while the satellites of our blue hosts are distributed isotropically around their hosts. This agrees well with previous studies in which only one or two satellites were identified for each host. In addition, we compute the pairwise clustering of the satellites and find a strong tendency (> 99.9% confidence level) for pairs of satellites to be located on the same side of their host, resulting in a lopsided satellite distribution. The signal is most pronounced for the satellites of blue hosts, where ~50% more satellite pairs are found on the same side of their host than are found on opposite sides. If the hosts’ dark matter halos are relaxed, the lopsided satellite distributions that we find suggest that satellite galaxies do not faithfully trace the shapes of their hosts’ dark matter halos. This could be an indication of late-time infall of satellites, particularly in the case of blue hosts. While lopsided distributions have been found previously for the satellites associated with pairs of bright galaxies, ours is the first study to reveal the existence of lopsided distributions for the satellites of isolated bright galaxies.