FORGE’d in Fragments: Pop III star formation in a Milky Way-like progenitor halo

We present a radiation-magnetohydrodynamic (RMHD) cosmological zoom-in simulation of Pop III star formation, drawing initial conditions from a Milky Way-like progenitor halo at redshift z ∼ 21. We follow the collapse of primordial gas over 10 orders of magnitude, through to the formation of an accretion disk that becomes gravitationally unstable Δt ∼ −3 kyr before the first protostar’s formation. Prominent spiral arms then develop and fragment vigorously, producing ∼60 sink formation events over the first 8 kyr post-collapse evolution, with a total stellar mass reaching >150 M_☉ by 15 kyr, the end of our simulation. The disk remains magnetically supercritical throughout: the mass-to-flux ratio falls from μ_Φ ∼ 10²–10³ to ∼15 by 15 kyr, as the inflow brings in magnetic flux. The magnetic energy in the disk remains sub-dominant throughout, but grows by ∼3.5 orders of magnitude after the first star forms. The disk starts sub-virial (2E_kin/|E_pot| ≈ 0.6) and reaches virial equilibrium within ∼2 kyr. The total stellar mass of the population grows approximately as M_⋆ ∝ t², confirming the prediction of accelerating accretion in self-gravitating turbulent collapse, at rates of Ṁ ∼ 10⁻³–10⁻² M_☉ yr⁻¹. The star formation efficiency reaches ≈90% within the disk (after ≈11 kyr; ≈80% at 9.5 kyr) and ∼50% within a 0.1 pc sphere. We find a top heavy IMF (dN/dm ∝ m^−α) with a high-mass slope α ≈ 1. The cosmological environment sets the boundary conditions of the disk: an envelope delivers ∼3 × 10⁻³ M_☉ yr⁻¹ to the outer edge of a disk with a radius of ∼10³ AU; the disk spreads and, in our simulation, its fragmentation sites move outward, from ∼500 to ∼1350 AU, until it has been depleted.

Submitted to ApJ