Abstract

Context. The clustering of dark-matter halos depends primarily on halo mass. However, when using a fixed halo mass, numerical simulations have been able to reveal multiple secondary dependencies. This so-called secondary halo bias has important implications for our understanding of structure formation and observational cosmology. Despite its significance, the effect has not yet been measured observationally with statistical confidence. Aims. We aim to develop the first observational method to probe halo spin bias: the secondary dependence of halo clustering on halo spin at a fixed halo mass. Methods. We used a proxy for halo spin based on the coherent motion of galaxies within and around a halo. We tested this technique using the IllustrisTNG hydrodynamical simulation and subsequently applied it to a group catalog from the Sloan Digital Sky Survey (SDSS). By splitting the SDSS groups according to this spin proxy and measuring the two-point correlation function of the resulting samples, we investigated the existence of halo spin bias. Results. We find consistent indications that at a fixed mass, groups with higher values for the spin proxy exhibit a higher bias than those with lower spin proxy values on scales of 5 15 h1 Mpc. The highest significance was observed for groups with halo masses Mh1013.2h1 M?, for which 85% of the sampled measurements display the expected trend. As we continue to improve on the method, our results could open new avenues for studying the connection between halo spin and the large-scale structure through the use of upcoming spectroscopic surveys. © The Authors 2025.