The Sheet of Giants: Unusual Properties of the Milky Way's Immediate Neighbourhood

TL;DR

This study characterizes the shape and density of the Local Sheet around the Milky Way, revealing it to be a flattened, overdense structure that is somewhat unusual compared to expectations from the $ ext{Lambda}$CDM model, with potential implications for galaxy formation.

Contribution

It provides the first detailed quantification of the Local Sheet's shape and density contrast, comparing observations with $ ext{Lambda}$CDM predictions and highlighting its unusual properties.

Findings

The Local Sheet is a flattened oblate ellipsoid with axis ratios c/a≈0.16 and b/a≈0.79.

The galaxy density in the Local Volume is 1.7 and 5.2 times higher than average for faint and bright galaxies.

The Local Volume's density contrast makes it a ~2.5σ outlier in $ ext{Lambda}$CDM}.

Abstract

We quantify the shape and overdensity of the galaxy distribution in the `Local Sheet' within a sphere of $R=8$ Mpc, and compare these properties with the expectations of the $\Lambda$CDM model. We measure ellipsoidal axis ratios of $c/a\approx0.16$ and $b/a\approx0.79$, indicating that the distribution of galaxies in the Local Volume can be approximated by a flattened oblate ellipsoid, consistent with the `sheet'-like configuration noted in previous studies. In contrast with previous estimates that the Local Sheet has a density close to average, we find that the number density of faint and bright galaxies in the Local Volume is $\approx1.7$ and $\approx5.2$ times denser, respectively, than the mean number density of galaxies of the same luminosity. Comparison with simulations shows that the number density contrasts of bright and faint galaxies within $8$ Mpc alone make the Local Volume a $\approx 2.5\sigma$ outlier in the $\Lambda$CDM cosmology. Our results indicate that the cosmic neighbourhood of the Milky Way may be unusual for galaxies of similar luminosity. The impact of the peculiar properties of our neighbourhood on the properties of the Milky Way and other nearby galaxies is not yet understood and warrants further study.