Spiral Structure Diversity in Milky Way Analogs from TNG50: The Role of Gas and Disk Dynamics

TL;DR

This study uses the TNG50 cosmological simulation to analyze the formation, properties, and physical drivers of spiral arms in Milky Way-like galaxies, revealing the roles of gas, disk coldness, and shear in spiral morphology.

Contribution

First systematic investigation of spiral-arm formation and properties in cosmological simulations, linking internal dynamics to galaxy assembly and observable features.

Findings

Most analogs show two-armed spirals in stars and gas.

Gas-rich and colder disks develop more prominent and tightly wound spirals.

Spiral pitch angle correlates with disk shear but with significant scatter.

Abstract

The generation of spiral arms and the mechanisms controlling their properties within a realistic cosmological framework - the complete understanding is still beyond our grasp. Using a statistically significant sample of Milky Way- and Andromeda-like (MW/M31) analogs from the high-resolution TNG50 cosmological simulation, we carry out the first systematic investigation of spiral-arm formation, their observable properties, and the underlying physical drivers. The selected analogs predominantly exhibit two-armed ($m = 2$) spirals in both stars and gas, while the gaseous disks often display stronger, more tightly wound, and multi-armed patterns ($m>2$). Spiral features appear across stellar populations of different ages, confirming their density-wave nature and producing coherent spirals in both metallicity and mean stellar age distributions-consistent with recent Gaia observations of the Milky Way. Our analysis reveals a diverse dynamical scenario for spiral generation: gas content, disk coldness, and shear jointly regulate the growth and morphology of spiral perturbations. We find that the gas content and the dynamical coldness of the disk jointly regulate spiral growth: galaxies with higher gas fractions and colder disks develop more prominent spirals. The measured relation between spiral pitch angle and disk shear shows significant scatter around the analytic prediction, likely due to the combined influence of bars, gas inflows, and feedback. These results demonstrate that spiral density waves can persist in fully cosmological disks, linking internal dynamical processes to galaxy assembly and offering testable predictions for present and future surveys such as JWST and Roman.