Galaxy Clusters from the DESI Legacy Imaging Surveys. IV. Alignment of Member Galaxy Orientations at z < 0.5

Abstract Using DESI photometric catalog data, we systematically analyze the spatial alignment of star-forming galaxies (SFGs) and quiescent galaxies (QGs) in clusters. The results reveal that, in both brightest central galaxy (BCG) alignment (BA) and satellite alignment (SA), the alignment signals of SFGs and QGs exhibit opposite trends in several aspects. For the BA signal, SFGs show stronger alignment in cluster outskirts, whereas QGs exhibit stronger alignment in inner regions. For the SA signal, both SFGs and QGs exhibit nearly isotropic signals in outer regions, while in inner regions they show opposite signals. In addition, the BA signal depends on cluster richness and QG fraction, and is more pronounced in mature clusters. Specifically, QGs show a stronger BA signal overall, which increases with luminosity and axis ratio, whereas SFGs show a weaker BA signal with no significant dependence on these parameters. The BA signal also exhibits a strong dependence on satellite galaxy color, which is independent of morphology. We confirm this by analyzing the BA signal in two populations: red disk and blue elliptical galaxies. However, neither BA nor SA shows a significant dependence on redshift at z < 0.5. In summary, these findings reflect how galaxy transport along cosmic filaments drives cluster evolution: early stage SFGs fall into clusters along filaments, and during cluster relaxation their spatial orientations are reshaped by gravitational potentials and tidal forces, weakening BA signal and enhancing SA signal. In contrast, QGs, as early cluster members, exhibit a more pronounced anisotropic distribution and maintain relatively stable spatial orientations.