Galaxy alignment on large and small scales

TL;DR

This paper investigates galaxy alignment on different scales using hydrodynamical simulations, revealing that alignments depend on galaxy properties, halo mass, and are influenced by galaxy-halo shape correlations rather than cosmic filaments.

Contribution

It provides a detailed analysis of galaxy alignment origins on small and large scales, emphasizing the role of galaxy-halo shape correlations and halo mass.

Findings

Red satellites align with central galaxy shapes within halos.

Alignment strength increases with halo mass and is stronger for red galaxies.

Large-scale alignment is partly due to galaxy-halo shape correlation, not cosmic filaments.

Abstract

Galaxies are not randomly distributed across the universe but showing different kinds of alignment on different scales. On small scales satellite galaxies have a tendency to distribute along the major axis of the central galaxy, with dependence on galaxy properties that both red satellites and centrals have stronger alignment than their blue counterparts. On large scales, it is found that the major axes of Luminous Red Galaxies (LRGs) have correlation up to 30Mpc/h. Using hydro-dynamical simulation with star formation, we investigate the origin of galaxy alignment on different scales. It is found that most red satellite galaxies stay in the inner region of dark matter halo inside which the shape of central galaxy is well aligned with the dark matter distribution. Red centrals have stronger alignment than blue ones as they live in massive haloes and the central galaxy-halo alignment increases with halo mass. On large scales, the alignment of LRGs is also from the galaxy-halo shape correlation, but with some extent of mis-alignment. The massive haloes have stronger alignment than haloes in filament which connect massive haloes. This is contrary to the naive expectation that cosmic filament is the cause of halo alignment.