From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?

Using FIRE-2 cosmological simulations, this paper demonstrates that the Milky Way's evolution through three distinct kinematic phases—from a disordered protogalaxy to a rotating thick disk and finally a thin disk—is driven primarily by the stabilization of the galaxy's center of mass and the virialization of the circumgalactic medium, rather than by major mergers.

The Gist

Imagine the Milky Way galaxy not as a static, perfect spiral, but as a living, breathing entity that grew up in three distinct "phases" of life, much like a child growing into a teenager and then an adult.

For a long time, astronomers have looked at the stars in our galaxy and realized they tell a story of three different eras. This paper uses powerful computer simulations to figure out why the galaxy changed from one era to the next.

Here is the story of the Milky Way's growth, explained simply:

Phase 1: The Chaotic Toddler (The Protogalaxy)

The Scene: In the beginning, the galaxy was a messy, disordered cloud of gas and stars. It was like a toddler running around a room with no rhythm, bumping into furniture, and spinning in random directions.
The Physics: The gas was sloshing around wildly. The center of the galaxy wasn't stable; it was wobbling back and forth. Because the "center" kept moving, the gas couldn't settle down to spin in a circle. It was just a chaotic mess.
The Result: Stars born during this time are like the toddler: they move in all directions, with no clear pattern.

Phase 2: The Spinning Teenager (The Thick Disk)

The Scene: Suddenly, the galaxy "spun up." The wobbling stopped, and the gas started to rotate in a coherent circle. However, it wasn't a flat, calm circle yet. It was a "thick" disk, like a spinning pizza dough that is still puffy and turbulent.
The Physics: The key trigger for this change wasn't a giant crash or a merger with another galaxy. Instead, it happened when the galaxy finally found its balance. The center of mass stopped sloshing around. Once the galaxy had a stable "center of gravity," the gas could finally start spinning together.
The Result: New stars formed in this phase are like energetic teenagers. They are all spinning in the same direction (coherent rotation), but they are still a bit wild, moving up and down and side-to-side, creating a "thick" disk.

Phase 3: The Calm Adult (The Thin Disk)

The Scene: Much later, the galaxy "cooled down." The puffy, turbulent disk flattened out into the beautiful, thin, crisp spiral we see today.
The Physics: This transition required more than just a stable center. It required the "atmosphere" around the galaxy (called the Circumgalactic Medium) to settle down. Think of it like a stormy ocean finally becoming calm. Once the gas surrounding the galaxy heated up and stabilized, it could rain down onto the galaxy slowly and gently. This slow, gentle rain allowed the gas to align perfectly before turning into stars.
The Result: New stars formed here are like calm adults. They sit perfectly flat in a thin disk, moving in smooth, orderly circles.

The Big Surprises

The paper found a few things that might surprise you:

• It wasn't a crash: We often think big changes in galaxies are caused by massive collisions (mergers). But this study found that these three phases happened naturally as the galaxy grew. Mergers didn't seem to be the main driver of these transitions.
• Gas leads, stars follow: The gas started spinning in Phase 2 before the new stars did. It's like the gas decided to dance first, and the stars just followed along a little later.
• The "Puffy" Center: You might think a galaxy needs a super-dense, heavy center to start spinning. The study found the opposite! The galaxy started spinning up when its center was actually quite "puffy" and spread out. It only became super-dense and concentrated after the spinning had already started.

The Takeaway

The Milky Way didn't evolve in one big leap. It went through a chaotic childhood, found its balance to become a spinning teenager, and finally settled into a calm adult state.

• To get the "Thick Disk" (Phase 2): You just need the galaxy to stop wobbling and find a stable center.
• To get the "Thin Disk" (Phase 3): You need the gas around the galaxy to calm down and rain in slowly, allowing everything to line up perfectly.

This research helps us understand that the galaxy's history is written in the motion of its stars, and that finding a stable center of gravity is the first step to becoming a beautiful, spinning disk.

Technical Summary

Problem Statement
Recent observational data from Gaia and APOGEE have revealed that the Milky Way's in situ stellar population evolved through three distinct kinematic phases: an early disordered protogalaxy, a kinematically hot thick disk, and a kinematically cold thin disk. While these phases are well-documented archaeologically, the physical mechanisms driving the transitions between them—specifically the "spin-up" from a protogalaxy to a thick disk, and the subsequent "cooldown" to a thin disk—remain to be fully understood. Previous work has suggested various drivers, including halo assembly rates, central potential concentration, and the virialization of the circumgalactic medium (CGM), but a unified physical explanation for the three-phase sequence in Milky Way-mass systems is lacking.

Methodology
The authors utilize a suite of 12 cosmological "zoom-in" simulations of Milky Way-mass galaxies from the Feedback In Realistic Environments (FIRE-2) project. These simulations employ the GIZMO code with mesh-free finite mass (MFM) hydrodynamics and include multi-channel stellar feedback (winds, supernovae, radiation pressure). The sample includes isolated halos and Local Group-like pairs.

The study employs an "archaeological" approach analogous to observational studies, mapping the orbital circularity ($\epsilon = j_z / j_c(E)$) of stars and gas against metallicity and lookback time. The authors define three characteristic transition times for each galaxy:

1. Gas Spin-up ($t_{s, \text{gas}}$): The time when the median circularity of cool gas ($T \le 10^4$ K) first rises sustainably above zero.
2. Stellar Spin-up ($t_{s, \star}$): The time when the median circularity of young stars ($< 250$ Myr) rises sustainably above zero.
3. Cooldown ($t_{\text{CD}}$): The time when the median circularity of young stars approaches unity ($\epsilon \approx 1$) with low dispersion, marking the onset of the thin disk phase.

The authors then correlate these transition times with various physical drivers, including virial mass, stellar mass, gas fraction, merger histories, the concentration of the central gravitational potential, the virialization state of the inner CGM, and the "sloshing" of baryonic matter relative to the total center of mass.

Key Results

1. Reproduction of Three Phases: The FIRE-2 simulations naturally reproduce the three kinematic phases observed in the Milky Way. All 12 galaxies exhibit an early isotropic protogalaxy phase, followed by a thick disk phase with coherent rotation but high dispersion, and finally a thin disk phase with high circularity and low dispersion.
2. Timing of Transitions:
   • Gas precedes Stars: In all cases, the gas spins up before the young stars (by $\sim$0.5 to >2 Gyr).
   • Spin-up vs. Cooldown: Stellar spin-up occurs significantly earlier than cooldown. The two transitions are not tightly correlated, suggesting distinct physical drivers.
3. Drivers of Spin-up (Protogalaxy $\to$ Thick Disk):
   • Not Mass or Mergers: Spin-up does not correlate with a specific virial mass, stellar mass, or gas fraction. Major mergers do not systematically trigger spin-up; in fact, spin-up often occurs after periods of rapid mass growth.
   • Not Potential Concentration: Contrary to some hypotheses, the central gravitational potential is least concentrated (rising rotation curves) at the time of spin-up. The potential steepens only later, closer to the cooldown phase.
   • Stable Center of Mass: The primary condition for spin-up is the cessation of "baryonic sloshing." The authors define a sloshing parameter ($s_b$) measuring the velocity offset between baryons and the total center of mass. Gas spin-up consistently occurs when $s_b$ drops below $\sim 0.2$, indicating that a stable dynamical center is required for coherent rotation to emerge.
4. Drivers of Cooldown (Thick Disk $\to$ Thin Disk):
   • CGM Virialization: The transition to the thin disk coincides with the virialization of the inner CGM, defined by the ratio of cooling time to free-fall time ($t_{\text{cool}}/t_{\text{ff}}$) exceeding $\sim 2$.
   • Potential Steepening: By the time of cooldown, the central potential has become significantly more concentrated.
   • Star Formation Mode: The thin disk phase is associated with a transition from bursty to steady star formation and intermediate rates of cool gas conversion into stars.
5. Gas Consumption Rate (GCR): The rate of cool gas conversion into stars peaks during the thick disk phase (between spin-up and cooldown), even while star formation remains bursty. After cooldown, the GCR decreases to intermediate levels as star formation becomes steady.

Significance and Conclusions
The paper concludes that the formation of a thick disk and a thin disk are driven by different physical conditions.

• Thick Disk Formation: Requires a minimal condition: a stable center of mass motion. Once the "sloshing" of baryons within the potential subsides, gas can begin to rotate coherently, even if the potential is still "puffy" and the inner CGM is not yet virialized.
• Thin Disk Formation: Requires more restrictive conditions. It necessitates the virialization of the inner CGM (allowing gas to accrete sub-sonically and align angular momentum) and a steepening of the central potential.

The authors emphasize that mergers do not appear to be the defining driver for either transition in their sample. The results suggest that while rotating gas alone is a sign of disk formation, the emergence of a thin disk specifically requires the thermodynamic and angular momentum structuring of the accreting gas provided by a virialized inner CGM. This distinction helps clarify the physical requirements for galaxy disk evolution in $\Lambda$CDM cosmology and offers a framework for interpreting high-redshift observations where gas rotation may exist without a settled thin stellar disk.