2025/12/19 by Qianyu Ma, Cheng Li, Ma, Qinglin +5
Physics and Astronomy · #Galaxies: Formation, Evolution, Phenomena #Astronomy and Astrophysical Research #Dark Matter and Cosmic Phenomena
paper · doi:10.48550/arxiv.2512.17742
We use the IllustrisTNG300-ODM simulation to investigate the spin bias of low-mass halos and its connection to the strong clustering of ultra-diffuse galaxies (UDGs) reported by Zhang et al. (2025). By comparing two halo spin definitions-one using only bound particles (λ\rm b) and another including unbound particles (λ\rm a)-we demonstrate that the spin bias of low-mass halos critically depends on the definition. While λ\rm a yields stronger clustering for higher-spin halos at all masses, λ\rm b produces an inverted trend below M\rm h∼ 1011 \rm M\odot/h. This discrepancy is driven by a subset of halos in high-density environments that have large λ\rm a but small λ\rm b. Using an empirical model implemented in SDSS-like mocks, we link the stellar surface-mass-density (Σ_∗) of a galaxy to λ\rm a of its host halo and find an anti-correlation that more diffuse dwarfs tend to reside in higher-spin halos. The model naturally reproduces the observed strong clustering of UDGs within the standard ΛCDM framework without invoking exotic assumptions such as self-interacting dark matter. The high fraction of unbound particles in UDG hosts likely originates from tidal fields in dense regions, an effect particularly significant for low-mass halos. We discuss how the angular momentum of a halo represented by λ\rm a may be transferred to the gas to affect size and surface density of the galaxy that forms in the halo.