Clues on void evolution – III. Structure and dynamics in void shells

Abstract

Inspired by the well-known dynamical dichotomy predicted in voids, where some underdense regions expand whereas others collapse due to overdense surrounding regions, we explored the interplay between the void inner dynamics and its large-scale environment. The environment is classified depending on its density as in previous works. We analyse the dynamical properties of void-centred spherical shells at different void-centric distances depending on this classification. The above dynamical properties are given by the angular distribution of the radial velocity field, its smoothness, the field dependence on the tracer density and shape, and the field departures from linear theory. We found that the velocity field in expanding voids follows more closely the linear prediction, with a more smooth velocity field. However, when using velocity tracers with large densities such deviations increase. Voids with sizes around 18 h−1 Mpc are in a transition regime between regions with expansion overpredicted and underpredicted from linear theory. We also found that velocity smoothness increases as the void radius, indicating the laminar flow dominates the expansion of larger voids (more than 18 h−1 Mpc). The correlations observed suggest that non-linear dynamics of the inner regions of voids could be dependent on the evolution of the surrounding structures. These also indicate possible scale couplings between the void inner expansion and the large-scale regions where voids are embedded. These results shed some light to the origin of non-linearities in voids, going beyond the fact that voids just quickly becomes non-linear as they become emptier.