Extragalactic void pushes Milky (a)Way

The presence of a large region in our extragalactic neighbourhood that is largely devoid of galaxies is reported in Nature Astronomy this week. This extragalactic void appears to be exerting a repelling force on our Local Group of galaxies, contributing to the gravitational forces affecting the Milky Way’s cosmic trajectory.

The presence of such a void has been suggested previously, but obtaining observational confirmation of the absence of galaxies has proved challenging. Yehuda Hoffman and colleagues instead studied the peculiar velocities - velocities in excess of the Universe’s rate of expansion - of galaxies around the Milky Way, combining different datasets of peculiar velocities with a rigorous statistical analysis of their properties. This investigation revealed that galaxies are flowing away from a void that the authors defined as the dipole repeller. Together with the gravitational pull of the previously known concentration of galaxies dubbed the Shapley attractor, the authors suggest that the two objects dominate the motion of the Milky Way.

The identification of the dipole repeller finally reconciles observations of both the direction and value of the Milky Way’s (about two million kilometres per hour) peculiar velocity. Future ultra-sensitive surveys at optical, near-infrared and radio wavelengths are required to directly identify the handful of galaxies expected to lie in this void and directly confirm the flow of galaxies away from empty regions and towards regions of high mass concentration.