Is the Milky Way Gravitationally Stable? A TNG50 View from the Cosmic Noon to the Present Day

Abstract

We investigate the stability of Milky Way analogs (MWAs) in the 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 (QT), the critical gas surface density (Σc), 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 QTmin < 2. The stability levels increase toward higher redshift, where enhanced gas velocity dispersion counterbalances the destabilizing effect of larger gas fractions. Furthermore, the barred MWAs consistently show lower QTmin than unbarred ones. The gas density remains subcritical (Σg < Σc) 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 disk, 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 (QT > 1), a combination of gas dissipation and turbulence in ISM can destabilize the disk at small scales, even when QT > 1.