2025ApJ...990....7S

We investigate early, z > 3, galaxy formation in a cosmological zoom-in simulation of a close, early-forming Milky Way (MW) analog extracted from TNG50 simulation and resimulated with detailed modeling of cold interstellar medium (ISM) formation, coupled with on-the-fly UV radiative transfer, turbulence-regulated star formation, and stellar feedback. In our enhanced-physics simulation, the galaxy develops a bistable ISM structure (warm, with T ∼ 10⁴ K, and cold, with T < 100 K) and exhibits significantly more efficient, early, and bursty star formation than in TNG. Notably, the stellar disk of this MW progenitor forms extremely early, around z ∼ 6-7, and exhibits chemo-kinematic properties consistent with the low-metallicity population of the MW stars. The disk forms rapidly, on a timescale of ∼0.2 Gyr, which is significantly shorter than the timescale implied by the observable chemo-kinematic signatures of disk spin-up, ∼0.7 Gyr, due to the scatter in the age-metallicity relation. The rotational support of the gas disk and the location of the galaxy on the main sequence are consistent with early disk galaxies observed by the James Webb Space Telescope and the Atacama Large Millimeter/submillimeter Array at z ∼ 4-7, suggesting that some of these galaxies could be progenitors of MW-like systems. Remarkably, the variation of the global star formation rate (SFR) before disk formation is similar to the observed SFR scatter at these early times. Our findings underscore the critical role of modeling a turbulent cold ISM and turbulence-regulated star formation and feedback in driving early SFR variability, while at the same time enabling early disk formation, without destroying it with overly efficient stellar feedback.