Walls, filaments and voids: it is thought that the large scale distribution of matter is a complex network of galaxies and galaxy clusters connected by elongated filaments and sheetlike walls, outlining vast underdense regions known as voids and meeting at dense and compact regions known as haloes. The image above depicts this: on the left we see the network of walls filaments and voids, in the center we see a zoom in on a denser region, and on the right we see a visualization of the residual velocity field in the cutout.
Aragon-Calvo et. al. in  build upon the SpineWeb framework outlined in  , which has the capability of identifying these walls, filaments and clusters in cosmological simulations to examine the effect of environment (namely whether a galaxy is “living” in a wall or a void) on the dispersion of the Hubble flow around the Milky Way, which is significantly lower than theoretical expectations. They show that the measured dispersion could be a result of the fact that Milky Way resides inside a wall of radius around 10Mpc, which is supported by data.
The above three figures depict the Distance-Velocity digram for Milky Way like haloes in a cosmological simulation which reside in walls considering three increasingly “cold” neighborhoods. The top left panel depicts contribution from all haloes around central haloes in walls. The top center panel is the same, but considers only those central haloes residing in a wall with density < 1.3. The top right panel depicts only the contribution from the haloes in the same wall as the central halo itself.
The bottom panels illustrate the difference between the dispersion in the simulation vs. that observed. Meaning the closer this is to 1, the more realistic the scenario seems. The middle panel is therefore the most realistic scenario. Moving back to the top panels and concentrating on the center panel, the key lines to consider are the solid white line (a pure Hubble flow with h=0.73) vs. the solid blue line (the Hubble flow in the simulation). We can see that for all three panels, the simulation flow is colder than the Hubble flow out to a given radius. Specifically in the center panel, the mean velocity dispersion is ~30km/s at 1 < r < 3 Mpc, which is said to match well with observations meaning that the Milky Way residing in such a wall could be a good candidate to explain this phenomenon.
1. M. A. Aragon-Calvo, J. Silk, & A. S. Szalay (2011). Locally Cold Flows from Large-Scale Structure MNRAS arXiv: 1103.1901v1
2. Miguel A. Aragon-Calvo, Erwin Platen, Rien van de Weygaert, & Alexander S. Szalay (2008). The Spine of the Cosmic Web The Astrophysical Journal, 723 (1) arXiv: 0809.5104v2