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

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.

Figures (12)

• Figure 1: Diverse spiral morphology in MW/M31 analogs: Face-on ($x-y$-plane) density distribution of the stars (first and third rows) and gas (second and forth rows) for a sample of non-barred (top two rows) and spirals+bar (bottom two rows) galaxies, considered for this work. $R_{\rm d}$ denotes the scale length of the stellar disk, and the values are taken from Pillepichetal2023. The color bars show the corresponding surface density (in logarithmic scale). The $\texttt{SubHaloID}$ is mentioned in each of the sub-panels. Both the non-barred and the spirals+bar samples show a wide variety in spiral morphology: from grand-design spirals to multi-armed flocculent spirals.
• Figure 2: Distribution of the most dominant spiral $m$ (arm multiplicity) for both stars (left panel) and gas (right panel). For further details, see the text. Non-barred galaxies are shown in blue while the spirals+bar galaxies are shown in yellow.
• Figure 3: Comparison of the spiral $m=2$ (top panel) and $m=4$ (bottom panel) arm strengths, for the stellar and the gas particles. The black dashed line in each panel denotes the 1:1 correspondence. The blue filled circles denote the spirals+bar galaxies whereas the yellow filled squares denote the non-barred galaxies from our selected MW/M31 analogs.
• Figure 4: Comparison of the average pitch angle ($\hbox{$\left<{i_m}\right>$}$) of the $m=2$ spirals (top panel) and the $m=4$ spirals (bottom panel), calculated separately for the stars and gas. The black dashed line in each panel denotes the 1:1 correspondence. The blue filled circles denote the spirals+bar galaxies whereas the yellow filled squares denote the non-barred galaxies from our selected MW/M31 analogs.
• Figure 5: Age dissection of spiral morphology: Face-on ($x-y$-plane) density distribution of the stars of different age bins for a sample of non-barred (left two rows) and spirals+bar (right two rows) galaxies, considered for this work. Top panels show for the young ($0 < \tau/\hbox{$\>{\rm Gyr}$} \leq 4$) stars, while middle and bottom panels show for the intermediate ($4 < \tau/\hbox{$\>{\rm Gyr}$} \leq 8$) and old ($8 < \tau/\hbox{$\>{\rm Gyr}$} \leq 12$) stars, respectively. $R_{\rm d}$ denotes the scale length of the stellar disk. The color bars show the corresponding surface density (in logarithmic scale). Spirals are present in all stellar populations of different age bins, indicating the density wave nature of spirals.