THE LOCAL GROUP IN THE COSMIC WEB

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 similar to 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 of 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: (1) preferentially located in filaments and sheets, (2) located in a narrow range of local overdensity 0 < delta < 2, web ellipticity 0.1 < e < 1.0, and prolateness -0.4 < p < 0.4, (3) 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, (e) over cap (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 (e) over cap (3). Additionally, we fail to find a strong correlation between the spin of each halo in the pair with the cosmic web. All of these trends are expected to a great extent from the selection of LG total mass in the general sample. Applied to the observed LG, there is a potential conflict between the alignments of the different satellite planes and the numerical evidence for satellite accretion along filaments; the direction defined by (e) over cap (3). This highlights the relevance of achieving a precise characterization for the location of the LG in the cosmic web in the cosmological context provided by Lambda CDM.