On the robustness of the velocity anisotropy parameter in probing the stellar kinematics in Milky Way like galaxies: Take away from TNG50 simulation

TL;DR

This study investigates the robustness of the velocity anisotropy parameter in stellar kinematics of Milky Way-like galaxies using TNG50 simulation data, highlighting how selection criteria influence observed profiles and proposing a new comparative method.

Contribution

It introduces a novel approach comparing radial and energy space profiles of the anisotropy parameter to assess sensitivity to selection criteria in galaxy simulations.

Findings

Halos with monotonic $eta$ profiles are mostly from class $ ext{A}$.

Profiles with peaks/troughs are from classes $ ext{B}$-$ ext{C}$.

Model diversity aligns with observational errors, supporting simulation validity.

Abstract

We analyze the velocity anisotropy of stars in real and energy space for a sample of Milky Way-like galaxies in the TNG50 simulation. We employ different selection criteria, including spatial, kinematic and metallicity cuts, and make three halo classes ($\mathcal{A}$-$\mathcal{C}$) which show mild-to-strong sensitivity to different selections. The above classes cover 48%, 16% and 36% of halos, respectively. We analyze the $\beta$ radial profiles and divide them into either monotonically increasing radial profiles or ones with peaks and troughs. We demonstrate that halos with monotonically increasing $\beta$ profiles are mostly from class $\mathcal{A}$, whilst those with peaks/troughs are part of classes $\mathcal{B}$-$\mathcal{C}$. This means that care must be taken as the observationally reported peaks/troughs might be a consequence of different selection criteria. We infer the anisotropy parameter $\beta$ energy space and compare that against the $\beta$ radial profile. It is seen that 65% of halos with very mild sensitivity to different selections in real space, are those for which the $\beta$ radial and energy profiles are closely related. Consequently, we propose that comparing the $\beta$ radial and energy profiles might be a novel way to examine the sensitivity to different selection criteria and thus examining the robustness of the anisotropy parameter in tracing stellar kinematics. We compare simulated $\beta$ radial profiles against various observations and demonstrate that, in most cases, the model diversity is comparable with the error bars from different observations, meaning that the TNG50 models are in good overall agreement with observations.