Spatial and velocity anisotropies of stellar halos across cosmic web environments: Insights from IllustrisTNG simulation

TL;DR

This study uses the IllustrisTNG simulation to examine how large-scale cosmic web environments influence the structural and kinematic anisotropies of stellar halos in Milky Way-mass galaxies, finding minimal environmental impact.

Contribution

It provides the first detailed analysis showing that cosmic web environment has little effect on stellar halo anisotropies at fixed halo mass, emphasizing stochastic formation processes.

Findings

No significant differences in anisotropy profiles across environments.

Stellar halos show increasing spatial anisotropy with radius.

Velocity anisotropy profiles are highly variable and locally diverse.

Abstract

The role of large-scale environment in shaping the structural and kinematic properties of stellar halos remains an open question. We investigate whether the cosmic web environments affect the spatial and velocity anisotropies of stellar halos in Milky Way-mass galaxies. Using high-resolution data from the TNG50 simulation, we analyze 29 stellar halos from each environment and quantify their spatial and kinematic anisotropies as a function of halo-centric radius. We find that stellar halos across all environments generally exhibit increasing spatial anisotropy with radius, with fluctuations corresponding to bound substructures. The velocity anisotropy profiles show radially dominated orbits on average, but also display significant local variation, including tangentially dominated regions. However, no statistically significant differences are observed in the mean spatial or velocity anisotropy profiles across environments, for either the total stellar halo population or for the in situ and ex situ components individually. The large scatter within each environment suggests that the formation of stellar halos is primarily driven by stochastic, small-scale processes such as satellite merger histories, rather than the large-scale geometry of the cosmic web. Our results imply that, at fixed halo mass, the influence of cosmic web environment on the structure of stellar halo is weak or highly non-deterministic. Possible environmental effects may be more prominent at higher masses where accretion is more anisotropic. Exploring this regime will require simulations with both larger volume and higher resolution.