Is Milky Way stable all the way ? A TNG50 view from cosmic noon to the present day

TL;DR

This paper investigates the stability of Milky Way analogs in the high-resolution TNG50 cosmological simulation from cosmic noon ($z=2.5$) to the present, using a two-component disc stability framework that includes stars, gas, and the dark matter halo to compute $Q_T$, $\Sigma_c$, and $\tau$ for 10 barred and 10 unbarred systems. The study finds that all MWAs remain stable against local axisymmetric instabilities with $Q_T^{\min}>2$, and that stability increases with redshift due to higher gas velocity dispersion; barred galaxies consistently exhibit lower $Q_T^{\min}$ and shorter $\tau$ in the inner disc, correlating with more centralized star formation. Across all epochs, the gas density stays subcritical ($\Sigma_g<\Sigma_c$), suggesting that local axisymmetric instabilities are not the primary driver of star formation, though gas dissipation and ISM turbulence can destabilize small scales even when $Q_T>1$. The results highlight self-regulation of the stellar+gas disc and the stabilizing role of the dark matter halo, while noting that non-axisymmetric processes and turbulence remain important channels for structure formation and star formation in MW-like systems.

Abstract

We investigate the stability of Milky Way analogs (MWAs) in the \texttt{TNG50} simulation against the growth of local axisymmetric instabilities, tracing their evolution from cosmic noon ($z=2.5$) to the present day ($z=0$). Using a two-component stability criterion that accounts for stars, gas, and the force field of the dark matter halo, we compute the net stability parameter ($Q_{T}$), the critical gas surface density ($Σ_{c}$), and the instability timescale ($τ$) for 10 barred and 10 unbarred MWAs. We find that these galaxies remain stable to axisymmetric instabilities at all epochs, with $Q_{T}^{\min}>2$. The stability levels increase toward higher redshift, where enhanced gas velocity dispersion counterbalances the destabilizing effect of larger gas fractions. Further, the barred MWAs consistently show lower $Q_{T}^{\min}$ than unbarred ones. The gas density remains subcritical ($Σ_{g}<Σ_{c}$) across radii and epochs, implying that local axisymmetric instabilities are not the primary channel for star formation. Growth timescales are short (a few Myr) in central regions but increase exponentially to several Gyr in the outer disc, naturally explaining the concentration of star formation toward galactic centers. We study the effect of gas dissipation and turbulence in ISM and find that while MWAs are stable against axisymmetric instabilities $(Q_{T}>1)$, a combination of gas dissipation and turbulence in ISM can destabilize the disc at small scales even when $Q_{T}>1$.

Figures (10)

• Figure 1: Face-on maps of stellar density [top] and gas density [bottom] for barred galaxies at $z=0$. The red color corresponds to the highest density, and the blue color corresponds to the lowest density.
• Figure 2: Face-on maps of stellar density [top] and gas density [bottom] for unbarred galaxies at $z=0$. The red color corresponds to the highest density, and the blue corresponds to the lowest density.
• Figure 3: Input parameters used to assess the stability of the unbarred subset of Milky Way analogs against local gravitational instabilities at various redshifts. In the first panel, we show the total circular velocity; in the second and third panels, we show radial velocity dispersion and surface density of stars and gas, respectively. The thick solid line shows the median profile of each parameter, while the thin lines represent individual subhalos.
• Figure 4: Input parameters used to assess the stability of the barred subset of Milky Way analogs against local gravitational instabilities at various redshifts. In the first panel, we show the total circular velocity; in the second and third panels, we show the radial velocity dispersion and the surface density of stars and gas, respectively. The thick solid line shows the median profile of each parameter, while the thin lines represent individual subhalos.
• Figure 5: We show the radial variation net stability levels of the galaxy $(Q_{T})$, and the stability of just stars $(Q_{\star})$ and gas $(Q_{g})$ across the disc of MWAs. The green curve depicts the stability of galaxies by excluding the stabilizing effect of the DM halo. In the top two rows, we show our results for unbarred MW analogs; in the bottom two panels, we show the results for the barred subsets.