Inferring the Morphology of Stellar Distribution in TNG50: Twisted and Twisted-Stretched shapes

TL;DR

This study analyzes the 3D shapes of stellar distributions in Milky Way-like galaxies from the TNG50 simulation, revealing twisting and stretching features, and compares their morphology with dark matter halos, showing significant alignment and the impact of substructures.

Contribution

Introduces a local in shell iterative method to characterize stellar morphology, revealing twisting and stretching in simulated galaxies and comparing these features with dark matter halos.

Findings

Over half of the halos show twisting in stellar distribution.

Dark matter and stellar shape profiles are closely aligned at small radii.

Substructures can influence orbital angular momentum but have a subdominant effect on overall shape.

Abstract

We investigate the morphology of the stellar distribution in a sample of Milky Way (MW) like galaxies in the TNG50 simulation. Using a local in shell iterative method (LSIM) as the main approach, we explicitly show evidence of twisting (in about 52% of halos) and stretching (in 48% of them) in the real space. This is matched with the re-orientation observed in the eigenvectors of the inertia tensor and gives us a clear picture of having a re-oriented stellar distribution. We make a comparison between the shape profile of dark matter (DM) halo and stellar distribution and quite remarkably see that their radial profiles are fairly close, especially at small galactocentric radii where the stellar disk is located. This implies that the DM halo is somewhat aligned with stars in response to the baryonic potential. The level of alignment mostly decreases away from the center. We study the impact of substructures in the orbital circularity parameter. It is demonstrated that in some cases, far away substructures are counter-rotating compared with the central stars and may flip the sign of total angular momentum and thus the orbital circularity parameter. Truncating them above 150 kpc, however, retains the disky structure of the galaxy as per initial selection. Including the impact of substructures in the shape of stars, we explicitly show that their contribution is subdominant. Overlaying our theoretical results to the observational constraints from previous literature, we establish fair agreement.