Hot or Cold? Radial Redistribution of Stars in FIRE Simulations of Milky Way-Mass Galaxies and the Asymmetry of Inward versus Outward Migrators

Technical Summary: Hot or Cold? Radial Redistribution of Stars in FIRE Simulations of Milky Way-Mass Galaxies

Problem Statement
Stellar radial redistribution (migration) within galactic disks complicates the reconstruction of galactic assembly history. While stellar abundances preserve a record of the interstellar medium (ISM) at birth, radial redistribution moves stars to galactocentric radii far from their formation sites, scrambling the link between present-day observables and birth conditions. A central debate in galactic archaeology concerns the dynamical nature of this redistribution: does it occur primarily via "cold torquing" (conserving orbital eccentricity while changing angular momentum) or "dynamical heating" (increasing eccentricity)? Previous theoretical works often rely on idealized models assuming stars form on circular orbits in settled disks, an assumption that may not hold for the turbulent, clumpy conditions of early galaxy formation. This paper addresses the lack of quantitative constraints on whether radial redistribution in cosmological contexts is dynamically "hot" or "cold."

Methodology
The authors analyze 12 Milky Way-mass galaxies ($M_{200m} = 1\text{--}2 \times 10^{12} M_\odot$) from the Feedback in Realistic Environments (FIRE) cosmological zoom-in simulations. The sample includes 6 galaxies from the Latte suite and 6 Local Group-like pairs from the ELVIS suite. Eleven galaxies utilize the FIRE-2 model, while one (m12q) utilizes the newer FIRE-3 model. These simulations self-consistently model the multiphase ISM, star formation, feedback, and the evolution of internal structures (spirals, bars) and external perturbations (mergers, satellites).

The analysis focuses on star particles that underwent "significant radial redistribution," defined as a fractional change in specific orbital angular momentum $|\Delta j_\phi / j_{\phi, \text{birth}}| > 0.2$. The authors quantify the dynamical nature of this redistribution by measuring the net change in orbital eccentricity ($\Delta e = e_{\text{now}} - e_{\text{birth}}$) between birth and the present day.

• Cold Torquing: Defined as significant angular momentum change with minimal eccentricity change ($|\Delta e| < 0.1$).
• Heating/Cooling: Defined as increases or decreases in eccentricity, respectively.
• Migration Direction: Stars are classified as inward migrators ($-1 < \Delta j_\phi / |j_{\phi, \text{birth}}| < 0$) or outward migrators ($\Delta j_\phi / |j_{\phi, \text{birth}}| > 0$ or $\Delta j_\phi / |j_{\phi, \text{birth}}| < -1$).

The study stratifies stars into three dynamical eras based on the disk's formation history: Pre-disk (dispersion-dominated), Early-disk (transitioning to rotation-dominated), and Late-disk (thin, rotation-dominated).

Key Results

1. Asymmetry of Migration Direction: The direction of radial redistribution is the primary determinant of dynamical heating.
   • Outward migrators experience significantly smaller net changes in eccentricity. They are the only population where the "cold-torqued" fraction remains substantial, particularly for younger stars.
   • Inward migrators almost always experience dynamical heating ($\Delta e > 0$) since birth, regardless of their birth epoch.
2. Dependence on Birth Eccentricity ($e_{\text{birth}}$):
   • Stars born on near-circular orbits ($e_{\text{birth}} < 0.2$) are the most susceptible to heating. For these stars, the cold-torqued fraction drops rapidly with age, falling below 50% for ages $\gtrsim 2$ Gyr.
   • Stars born on moderately eccentric orbits ($e_{\text{birth}} \approx 0.4\text{--}0.6$) are the most likely to preserve their birth eccentricity (i.e., be cold torqued).
   • Stars born on highly eccentric orbits (typical of the pre-disk era) largely preserve their high eccentricity simply because they occupy the upper range of available phase space and cannot be heated further.
3. Dynamical Cooling: A significant population of stars has dynamically "cooled" (decreased in eccentricity) since birth. This cooling is the primary mechanism by which stars end up on near-circular orbits today, particularly for outward migrators born in the early- or late-disk eras.
4. Age Dependence: For stars born on near-circular orbits, the cold-torqued fraction declines sharply with age. Only the youngest outward migrators ($\lesssim 1$ Gyr) show a cold-torqued fraction exceeding 50%. Older stars, even those born in the late-disk era, rarely preserve their initial circularity.
5. Disk Evolution Context: The results vary by formation era. Pre-disk stars (born in turbulent ISM) show broad distributions of redistribution but generally maintain their high birth eccentricities. Late-disk stars show the strongest sensitivity to migration direction, with inward migration causing rapid heating and outward migration allowing for cooling or preservation of circularity.

Significance and Claims
The paper claims to be the first to quantify the "hot" versus "cold" nature of radial redistribution in cosmological simulations of Milky Way-mass galaxies that self-consistently model the key drivers of redistribution.

• Challenge to Idealized Models: The findings contradict the common assumption that radial redistribution is predominantly a "cold" process preserving orbital circularity. In the FIRE simulations, radial redistribution between birth and today is typically not cold, especially for inward migrators and older stellar populations.
• Reinterpretation of Observables: The authors argue that the present-day orbital circularity of a star is not a reliable proxy for its birth conditions. Stars on near-circular orbits today often arrived there via dynamical cooling (outward migration), not by preserving a circular birth orbit. Conversely, stars that migrated inward are almost certainly heated, regardless of their birth orbit.
• Galactic Archaeology Implications: The study suggests that inferring birth conditions from present-day properties requires accounting for migration direction and birth eccentricity. The "cold torquing" fraction is highly sensitive to the specific dynamical era of formation and the direction of migration, implying that simple models assuming universal cold migration may misinterpret the assembly history of the Milky Way.

The authors conclude that a star's migration direction, its birth eccentricity, and its age are the primary determinants of whether it was dynamically heated, cooled, or unchanged, with the general trend in FIRE simulations being that radial redistribution is a dynamically hot process over stellar lifetimes.