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

Using the TNG50 simulation, this study finds that Milky Way analogs from cosmic noon to the present day remain globally stable against axisymmetric instabilities due to high stability parameters and velocity dispersion, although local turbulence and dissipation can still induce small-scale instabilities, particularly in barred galaxies.

Original authors: K. Aditya, Sandeep Kataria

Published 2026-01-28

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CC BY 4.0

Original authors: K. Aditya, Sandeep Kataria

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Milky Way not as a static picture, but as a living, breathing dance floor that has been spinning for billions of years. This paper asks a simple but profound question: Is this dance floor stable, or is it about to collapse into a chaotic pile of stars and gas?

The authors, K. Aditya and Sandeep Kataria, used a super-powerful computer simulation called "TNG50" to rewind the clock on 20 galaxies that look just like our own Milky Way. They watched these galaxies evolve from "Cosmic Noon" (a time when the universe was about half its current age and very active) to the present day.

Here is what they found, explained through everyday analogies:

1. The "Stability Score" (The Toomre Q)

Think of a galaxy's disk like a spinning pizza dough. If you spin it too slowly, gravity pulls the dough inward, and it collapses. If you spin it fast enough, or if the dough is "hot" (moving around wildly), it stays flat and stable.

The scientists calculated a "Stability Score" (called $Q_T$ ) for these galaxies.

The Rule: If the score is below 1, the pizza collapses. If it's above 1, it holds its shape.
The Finding: Every single galaxy they studied had a score well above 2. This means the Milky Way and its cousins have been remarkably stable, never on the verge of collapsing due to gravity, from the early universe until today.

2. The "Heat" vs. The "Crowd"

You might think that in the early universe, galaxies were unstable because they were packed with so much gas (the "crowd"). Usually, a crowded room is chaotic.

The Twist: In the early universe, the gas wasn't just crowded; it was also incredibly "hot" and turbulent (moving very fast in random directions).
The Analogy: Imagine a mosh pit. If everyone is just standing still, they might crush each other (collapse). But if everyone is jumping and running wildly (high velocity), they actually keep each other apart. The "heat" of the gas counterbalanced the "crowd," keeping the galaxy stable even when it was very young.

3. The Barred vs. Unbarred Difference

Some of these galaxies had a "bar" (a straight line of stars running through the center, like a dumbbell), while others were round like a fried egg.

The Finding: The "barred" galaxies were slightly less stable than the round ones, but still very safe (score > 2).
The Metaphor: Think of the bar as a traffic jam in the center of the galaxy. It makes the center a bit more crowded and prone to movement, but not enough to cause a crash. However, because they are slightly less stable, they actually form stars a bit faster in the center, like a slightly more active dance floor.

4. Why Stars Form in the Center, Not the Edges

The paper looked at how long it takes for gravity to turn gas into stars.

The Center: In the middle of the galaxy, this process is fast—taking just a few million years. It's like a fast-food kitchen churning out burgers quickly.
The Edges: Out in the far reaches of the galaxy, it takes billions of years. It's like a slow-cooking stew.
The Result: This explains why our galaxy (and others) has a bright, busy center and a quiet, sparse edge. The "instability" that creates stars simply happens too slowly in the outer regions to make much of a difference.

5. The "Hidden Danger" (Gas Dissipation and Turbulence)

This is the most surprising part. The paper says the galaxies are "stable" according to the big rules ( $Q > 1$ ). But, the authors found a loophole.

The Analogy: Imagine a building that is structurally sound (stable). However, if you start drilling small holes in the walls (gas dissipation) or shaking the foundation with a specific frequency (turbulence), the building can still develop cracks, even if the main beams are fine.
The Finding: Even though the galaxies are stable against big, global collapses, the gas inside them is messy. If the gas loses its energy (dissipates) or gets turbulent, it can still break apart into small clumps and form stars. So, the galaxy is "safe" from a total crash, but "unsafe" from forming new stars in small pockets.

6. The Self-Regulating Machine

Finally, the authors discovered that these galaxies are like self-regulating thermostats.

Even if you pretend the invisible "Dark Matter" halo (the invisible glue holding the galaxy together) doesn't exist, the stars and gas still manage to keep the galaxy stable on their own.
The Metaphor: It's like a group of dancers who, even without a stage manager, instinctively adjust their speed and spacing so they don't bump into each other. The galaxy naturally balances its own density and speed to stay safe.

Summary

The Milky Way and its cosmic cousins have been gravitationally stable for billions of years. They didn't collapse because the gas was too "hot" and moving too fast in the early days. While they are safe from a total structural collapse, they are still active enough to form stars in their centers, driven by small-scale turbulence and energy loss in the gas, rather than a massive gravitational failure.

Technical Summary: Is Milky Way gravitationally stable? A TNG50 view from cosmic noon to the present day

Problem Statement
Local gravitational instabilities are fundamental to galaxy formation, driving gas fragmentation and star formation. While theoretical frameworks exist to quantify disc stability (e.g., Toomre $Q$ ), their application to the continuous evolution of Milky Way Analogs (MWAs) from cosmic noon ( $z \approx 2.5$ ) to the present day ( $z=0$ ) requires high-resolution cosmological simulations. A key question remains: Are MWAs stable against axisymmetric instabilities throughout their history, and how do factors like dark matter, gas fractions, and bar structures influence this stability? Furthermore, does the standard stability criterion ( $Q_T > 1$ ) fully capture the conditions for star formation, or do smaller-scale processes like gas dissipation and turbulence play a destabilizing role even in "stable" discs?

Methodology
The authors utilize a sample of 20 MWAs (10 barred and 10 unbarred) from the TNG50 cosmological simulation suite. These galaxies are selected based on disky morphologies, stellar masses ( $10^{10.5} - 10^{11.2} M_\odot$ ), and Milky Way-like environments at $z=0$ . The study traces their evolution from $z=2.5$ to $z=0$ .

The analysis employs a two-component dynamical model accounting for stars, gas, and the dark matter halo. Key quantities computed include:

Net Stability Parameter ( $Q_T$ ): Calculated using a criterion derived by K. Aditya (2024) that explicitly incorporates the dark matter halo's force field and the gas fraction. This differs from simpler approximations (e.g., inverse sum of components) by solving for the most unstable wavenumber ( $k_{min}$ ) semi-analytically.
Critical Gas Surface Density ( $\Sigma_c$ ): The threshold above which the gas disc becomes locally unstable.
Instability Growth Timescale ( $\tau$ ): The time required for instabilities to convert gas into stars.
Star Formation Rate (SFR): Derived directly from TNG50 snapshots.

The study also investigates the effects of gas dissipation and turbulence in the Interstellar Medium (ISM) by modifying the stability function to include scale-dependent terms, testing whether discs can become unstable at small scales even when $Q_T > 1$ .

Key Results

Global Stability: MWAs in TNG50 remain stable against local axisymmetric instabilities at all epochs ( $z=2.5$ to $z=0$ ). The minimum net stability parameter ( $Q_T^{min}$ ) consistently exceeds 2.
Redshift Evolution: Stability levels increase toward higher redshifts. Although gas fractions are higher at $z=2.5$ , this destabilizing effect is counterbalanced by significantly higher gas and stellar velocity dispersions and the stabilizing influence of the dark matter halo.
Barred vs. Unbarred: Barred galaxies consistently exhibit lower $Q_T^{min}$ values compared to unbarred galaxies. Consequently, barred systems show shorter instability growth timescales and higher central star formation rates.
Subcritical Gas: The gas surface density ( $\Sigma_g$ ) remains subcritical ( $\Sigma_g < \Sigma_c$ ) across all radii and epochs. This implies that local axisymmetric instabilities are not the primary driver of star formation in these simulations.
Timescale Gradient: The growth timescale $\tau$ increases exponentially with galactocentric radius. In central regions, instabilities can convert gas to stars in a few Myr, whereas in the outer disc, this process requires several Gyr. This naturally explains the concentration of star formation in galactic centers.
Self-Regulation: Even when the stabilizing contribution of the dark matter halo is removed ( $R=0$ ), the combined stellar and gas disc remains stable ( $Q_T > 1$ ). This indicates that stellar and gas components self-regulate their surface densities and velocity dispersions to maintain stability.
Small-Scale Instabilities: While the discs are stable against large-scale axisymmetric modes ( $Q_T > 1$ ), the inclusion of gas dissipation and scale-dependent turbulence (following Larson-type scaling relations) can destabilize the disc at smaller scales. Specifically, gas dissipation removes pressure support, and turbulence can enhance self-gravity scaling, driving instabilities even when $Q_T > 1$ .

Significance and Claims
The paper claims that MWAs in the TNG50 simulation are dynamically robust against axisymmetric instabilities from cosmic noon to the present day, driven by a self-regulating mechanism between stars and gas, reinforced by dark matter. The authors argue that the high stability levels ( $Q_T > 2$ ) explain why local axisymmetric instabilities are not the sole trigger for star formation in these galaxies.

However, the study highlights a critical nuance: the standard stability criterion may be insufficient to describe the full picture of disc dynamics. The authors demonstrate that gas dissipation and ISM turbulence can induce instabilities at small scales even in globally stable discs ( $Q_T > 1$ ). This suggests that while the large-scale structure of MWAs is stable, the internal ISM physics allows for continued fragmentation and star formation through mechanisms that bypass the traditional Toomre limit. The results also underscore the importance of multi-component stability analyses that include dark matter, as simulations that underpredict stellar-to-halo mass fractions may produce overly stable discs.

The work provides a continuous evolutionary link between high-redshift progenitors and local MWAs, showing that despite higher gas fractions in the early universe, enhanced dynamical support prevents the onset of global gravitational collapse.