The Anisotropic Distribution of Satellite Galaxies

Contrary to several recent studies, we find strong evidence in the Sloan Digital Sky Survey for the “Holmberg Effect”, the tendency for the satellites of isolated late-type galaxies to align with the minor axis of the galactic disc. In the case of isolated earlytypes, we find that satellites tend to align with the major axis of the galaxy. Using mock catalogues generated from cosmological N -body simulations, we demonstrate that previous studies, which found results to the contrary, used galaxy samples that were dominated by the members of galaxy groups rather than isolated galaxies. If we duplicate the selection criteria of these studies, we recover their results: namely, group members tend to cluster along the major axis of the brightest group galaxy if it is an early-type, but are distributed isotropically if it is a late-type. We find strong evidence for an anisotropic satellite distribution out to 350 h kpc around isolated late-types and 500 h kpc around isolated early-types. It is interesting to note that the polar alignment we detect around external late-types has the same sign and magnitude as the alignment of satellites around the major Local Group spirals. Our analysis suggests that satellites at large projected radii show a slight tendency to lie along the same axis as the surrounding large scale structure. We discuss the origin of the anisotropic satellite distribution and we consider the implications of our results, critically assessing the respective roles played by dynamical evolution of satellite orbits; anisotropic accretion of satellites from the larger scale environment; and the biased nature of satellites as tracers of the underlying dark matter subhalo population.