Velocity anti-correlation of diametrically opposed galaxy satellites in the low redshift universe

Abstract

Recent work has shown that both the Milky Way and the Andromeda galaxies possess the unexpected property that their dwarf satellite galaxies are aligned in thin and kinematically coherent planar structures. It is now important to evaluate the incidence of such planar structures in the larger galactic population, since the Local Group may not be a sufficiently representative environment. Here we report that the measurement of the velocity of pairs of diametrically opposed galaxy satellites provides a means to determine statistically the prevalence of kinematically coherent planar alignments. In the local universe (redshift $z<0.05$), we find that such satellite pairs out to a galactocentric distance of $150$ kpc are preferentially anti-correlated in their velocities (99.994% confidence level), and that the distribution of galaxies in the larger scale environment (beyond 150 kpc and up to $\approx 2$ Mpc) is strongly elongated along the axis joining the inner satellite pair ($>7\sigma$ confidence). Our finding may indicate that co-rotating planes of satellites, similar to that seen around the Andromeda galaxy, are ubiquitous in nature, while their coherent motion also suggests that they are a significant repository of angular momentum on $\sim 100$ kpc scales.