THE LOPSIDED DISTRIBUTION OF SATELLITE GALAXIES

The distribution of smaller satellite galaxies around large central galaxies has attracted attention because peculiar spatial and kinematic configurations have been detected in some systems. A particularly striking example of such behavior is seen in the satellite system of the Andromeda galaxy, where around 80% are on the near. side of that galaxy, facing the Milky Way. Motivated by this departure from anisotropy, we examined the spatial distribution of satellites around pairs of galaxies in the Sloan Digital Sky Survey. By stacking tens of thousands of satellites around galaxy pairs, we found that satellites tend to bulge toward the other central galaxy, preferably occupying the space between the pair, rather than being spherically or axis-symmetrically distributed around each host. The bulging is a function of the opening angle examined and is fairly strong-there are up to similar to 10% more satellites in the space between the pair. than expected from uniform. Consequently, it is a statistically very strong signal, being inconsistent with a uniform distribution at the 5 sigma level. The possibility that the observed signal is the result of the overlap of two halos with extended satellite distributions. is ruled out by testing this hypothesis by performing the same tests on isolated galaxies (and their satellites) artificially placed at similar separations. These findings highlight the unrelaxed and interacting nature of galaxies in pairs.