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

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.

Figures (5)

• Figure 1: The three panels show the spatial distribution of TNG50 galaxies, color-coded by cosmic web environment, along with the positions of 198 stellar halos from Milky Way/Andromeda-like galaxy catalogue. The circles representing stellar halos are visually enlarged for clarity and their sizes do not reflect the actual physical extent of the halos.
• Figure 2: The top three panels of this figure shows the radial variation of the whole-sky anisotropy in a set of stellar halos (29) residing in sheets, filaments and clusters respectively. The dotted line in each panel displays the mean whole-sky anisotropy of the 29 stellar halos in respective environment. The middle and bottom three panels show the results for the in situ and ex situ components of the same stellar halos. The same colour is used for the in situ, ex situ and combined components of each stellar halo within a given environment. We use $N_{\text{side}} = 32$ for the pixelization of the sky in each stellar halo in all environments.
• Figure 3: The left panel of this figure compares the mean whole-sky anisotropy parameter as a function of radial distance from the centre of stellar halos residing in sheets, filaments, and clusters. The middle panel presents results for the in situ component, while the right panel shows those for the ex situ component of the stellar halo. The 1$\sigma$ error bars shown at each data point are computed using whole-sky anisotropy measurements of 29 halos from each environment.
• Figure 4: The top three panels of this figure show the variation of the velocity anisotropy parameter ($\beta$) with radial distance from the centre of the 29 stellar halos in sheets, filaments and clusters respectively. The middle and bottom three panels show the same but for the in situ and ex situ components of stellar halo.
• Figure 5: The left panel compares the mean velocity anisotropy parameter $\beta$ as a function of radial distance from the centre of stellar halos residing in three different cosmic web environments (sheet, filament, cluster). The middle and right panels display the corresponding results for the in situ and ex situ components of the stellar halo, respectively. The 1$\sigma$ error bars shown at each data points are obtained using the velocity anisotropy measurements of 29 halos from each environment.