The lopsided distribution of satellite galaxies

TL;DR

This study reveals a significant bulging of satellite galaxies towards the space between galaxy pairs, indicating unrelaxed, interacting systems, based on analysis of SDSS data showing a strong, statistically significant anisotropic distribution.

Contribution

It demonstrates a novel, statistically significant anisotropic satellite distribution pattern around galaxy pairs, challenging the assumption of uniform satellite dispersal.

Findings

Satellites tend to bulge towards the space between galaxy pairs.

The bulging effect is up to 10% more satellites than expected from uniform distribution.

The observed anisotropy is statistically significant at the 5σ level.

Abstract

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 nearside 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 SDSS. By stacking tens of thousands of satellites around galaxy pairs we found that satellites tend to bulge towards 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 $\sim$10\% more satellites in the space between the pair, than expected from uniform. Consequently, it is a statistically very strong signal, being inconsistent with a uniform distribution at the 5$\sigma$ level. The possibility that the observed signal is the result of the overlap of two haloes 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.