Observational Evidence for Spin Alignment between Galaxy Groups and Their Central Galaxies

We present a systematic observational study of the spin alignment between galaxy groups and their central galaxies using a large spectroscopic sample from the Sloan Digital Sky Survey. Unlike previous studies based on morphology or major axis alignment, we use spin as a direct, dynamically motivated probe. To match the limitations of observational data, the group spin, central-galaxy spin, and alignment angle are defined and measured in projection on the sky plane. By applying two novel spin estimators for galaxy groups, we find strong statistical evidence for a preferential alignment, with a mean projected angle of 34 17 ± 0 29, significantly deviating from random expectation at 37.77σ. This alignment signal persists across a wide range of group and galaxy properties, but its strength is modulated by mass, morphology, and color. Specifically, we find that more massive groups and more massive central galaxies exhibit stronger alignment. Furthermore, elliptical central galaxies show stronger alignment than spirals, and bluer central galaxies are more strongly aligned than redder ones. Our results suggest a close dynamical link between the spin of central galaxies and their host groups, modulated by their physical properties and star formation history. These results provide new insights into the dynamical connection between central galaxies and their host dark matter halos.