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

TL;DR

This study uses simulations to explore how halo spin bias influences the clustering of ultra-diffuse galaxies, revealing that the definition of halo spin critically affects observed trends and that UDGs are linked to higher-spin halos.

Contribution

It demonstrates that the strong clustering of UDGs can be explained by halo spin bias within the standard cosmological model, depending on the halo spin definition used.

Findings

Different halo spin definitions lead to contrasting clustering trends.

UDGs tend to reside in higher-spin halos with more diffuse stellar distributions.

The model reproduces observed UDG clustering without exotic physics.

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 ($\lambda_{\rm b}$) and another including unbound particles ($\lambda_{\rm a}$)-we demonstrate that the spin bias of low-mass halos critically depends on the definition. While $\lambda_{\rm a}$ yields stronger clustering for higher-spin halos at all masses, $\lambda_{\rm b}$ 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 $\lambda_{\rm a}$ but small $\lambda_{\rm b}$. Using an empirical model implemented in SDSS-like mocks, we link the stellar surface-mass-density ($\Sigma_\ast$) of a galaxy to $\lambda_{\rm a}$ 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 $\Lambda$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 $\lambda_{\rm a}$ may be transferred to the gas to affect size and surface density of the galaxy that forms in the halo.