The Local Group in the cosmic web

TL;DR

This study investigates the cosmic web environment of Local Group-like pairs using simulations, revealing their preferential locations, alignments, and the importance of cosmic web characterization for understanding the LG in ΛCDM cosmology.

Contribution

It provides a detailed analysis of the cosmic web around LG-like pairs, including their preferred locations, orientations, and the relationship between their properties and the web structure, using a novel classification method.

Findings

LG-like pairs are mainly found in filaments and sheets.

Pairs show strong alignment with the cosmic web, especially in their orbital angular momentum.

No strong correlation found between halo spin and the cosmic web.

Abstract

We explore the characteristics of the cosmic web around Local Group(LG) like pairs using a cosmological simulation in the $\Lambda$CDM cosmology. We use the Hessian of the gravitational potential to classify regions on scales of $\sim 2$ Mpc as a peak, sheet, filament or void. The sample of LG counterparts is represented by two samples of halo pairs. The first is a general sample composed by pairs with similar masses and isolation criteria as observed for the LG. The second is a subset with additional observed kinematic constraints such as relative pair velocity and separation. We find that the pairs in the LG sample with all constraints are: (i) Preferentially located in filaments and sheets, (ii) Located in in a narrow range of local overdensity $0<\delta<2$, web ellipticity $0.1<e<1.0$ and prolateness $-0.4<p<0.4$. (iii) Strongly aligned with the cosmic web. The alignments are such that the pair orbital angular momentum tends to be perpendicular to the smallest tidal eigenvector, $\hat{e}_3$, which lies along the filament direction or the sheet plane. A stronger alignment is present for the vector linking the two halos with the vector $\hat{e}_3$. Additionally, we fail to find a strong correlation of the spin of each halo in the pair with the cosmic web. All these trends are expected to a great extent from the selection on the LG total mass on the general sample. Applied to the observed LG, there is a potential conflict between the alignments of the different planes of satellites and the numerical evidence for satellite accretion along filaments; the direction defined by $\hat{e}_3$. This highlights the relevance of achieving a precise characterization of the place of the LG in the cosmic web in the cosmological context provided by $\Lambda$CDM.