Interpreting the strong clustering of ultra-diffuse galaxies by halo spin bias

Abstract

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 $M_{\rm h}\sim 10^{11} \rm M_{\odot}/h$. 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 ($Σ_\ast$) 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 $Λ$CDM 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.