The Cosmic Dance: Observational Detection of Coherent Spin in Galaxy Clusters

Abstract The spin of galaxy clusters encodes key information about their formation, dynamics, and the influence of large-scale structure. However, whether clusters possess statistically significant spin and how to measure it observationally remain open questions. Here, we present the first observational, statistical detection of coherent spin in galaxy clusters, using two samples of 2170 and 1329 systems with M &gt; 10 14 M ⊙ , selected from two publicly available group catalogs constructed with two different algorithms and but both based primarily on Sloan Digital Sky Survey galaxies. Cluster spin is quantified by identifying the orientation in the projected plane that maximizes the redshift difference ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msub> <mml:mrow> <mml:mi>Z</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> </mml:math> ) between member galaxies in two regions divided by a trial axis. We find compelling statistical evidence for coherent rotation, as the observed <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:msub> <mml:mrow> <mml:mi>Z</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">max</mml:mi> </mml:mrow> </mml:msub> </mml:math> distribution departs markedly from the randomized controls, exhibiting pronounced deviations near 380 km s −1 . Stacked visualizations confirm the spatial segregation of redshifted and blueshifted galaxies across the rotation axis. The radial profile of the rotational velocity indicates that it increases as a function of radius. The cluster rotation speed increases with mass, from ∼360 km s −1 at 10 14 M ⊙ to ∼693 km s −1 at 10 15 M ⊙ . Additionally, cluster spin tends to align parallel with the central galaxy spin and perpendicular to the nearest cosmic filament, particularly in richer systems. These results reveal significant coherent spin in galaxy clusters, shaped by both internal dynamics and large-scale structure.