The Unusual Milky Way-Local Sheet System: Implications for Spin Strength and Alignment

TL;DR

This study uses cosmological simulations to explore the unique environment of the Milky Way and the Local Sheet, revealing rare alignments and low spin parameters that influence galaxy evolution understanding.

Contribution

It identifies the rarity of Milky Way analogues in Local-Sheet-like environments and highlights the environmental effects on galaxy spin and alignment.

Findings

MW analogues in Local-Sheet environments are rare.

Strong alignments between galaxy spins and the sheet are observed.

Galaxies in sheets tend to have low spin parameters.

Abstract

The Milky Way and the Local Sheet form a peculiar galaxy system in terms of the unusually low velocity dispersion in our neighbourhood and the seemingly high mass of the Milky Way for such an environment. Using the TNG300 simulation we searched for Milky Way analogues (MWA) located in cosmological walls with velocity dispersion in their local Hubble flow similar to the one observed around our galaxy. We find that MWAs in Local-Sheet analogues are rare, with one per (160-200 Mpc)^3 volume. We find that a Sheet-like cold environment preserves, amplifies, or simplifies environmental effects on the angular momentum of galaxies. In such sheets, there are particularly strong alignments between the sheet and galaxy spins; also, these galaxies have low spin parameters. These both may relate to a lack of mergers since wall formation. We hope our results will bring awareness of the atypical nature of the Milky Way-Local Sheet system. Wrongly extrapolating local observations without a full consideration of the effect of our cosmic environment can lead to a Copernican bias in understanding the formation and evolution of the Milky Way and the nearby Universe.