FORGE’d In The Early Universe II: Constraining The Pop III Initial Mass Function

Nov 5, 2026·

Yasmine J. Meziani

Philip F. Hopkins

Pratik J. Gandhi

Claude-André Faucher-Giguère

Michael Y. Grudić

Shivan Khullar

Vasilii Pustovoit

Cite DOI arXiv ADS

Figure 1. Visualizations of the gas mass and gas kinetic energy surface densities in the cosmological simulation at different redshifts throughout its evolution. Panels shown in purple represent the gas mass surface density, while the top-right panel in red indicates the kinetic energy surface density. Darker colors are regions of lower density, while brighter colors indicate higher density. Multiple panels are used to illustrate different stages of the simulation and to capture its wide dynamic range. The panels show selected regions of the simulation at different spatial scales and evolutionary times. The top-left panel shows a dense gas filament in which stars have formed, with feedback from the newly formed stars evacuating the surrounding gas and producing an evacuated cavity. Star formation occurs along dense gas filaments and within molecular clouds. Protostellar jets are visible before the stars reach the main sequence (top right). The middle-right panel shows a molecular cloud containing filamentary structures on a larger spatial scale. At later times, as the stars evolve and undergo supernovae, supernova remnants appear in the simulation (bottom left). The dots in the bottom-right panel indicate the positions of sink particles.

Abstract

We present a cosmological radiation-magnetohydrodynamics (RMHD) zoom-in simulation of 56 Population III stars using FORGE’d in FIRE. Our simulation combines a cosmological initial condition centered on an isolated ultra-faint galaxy (M_halo ∼ 10⁹ M_☉ at z = 0) with individual-star formation and feedback physics, reaching a maximum mass resolution of ∼10⁻² M_☉ and spatial resolution of ∼10⁻³ pc. We follow continued star formation for just over 500 Myr, from z ∼ 16 to z ∼ 7, substantially longer than what has been achieved before, and track star formation across five distinct minihalos. From our stellar population, we self-consistently resolve an emergent stellar initial mass function (IMF) and find α = 1.9 for dN/dM ∝ M^−α at M_⋆ ≳ 2.2 M_☉. Additionally, we form both single and multiple stellar systems, including binaries and higher-order multiples, as well as stars with masses below 1 M_☉. At early times, the stellar mass distribution is more top-heavy, with a high-mass slope of α ∼ 1.4–1.5, similar to previous Pop. III simulations, and steepens as star formation continues. This suggests that differences between our final IMF and previous studies may partly reflect the much longer evolution followed here, approximately five orders of magnitude longer than in several previous Pop. III sink-particle simulations. The first star-forming minihalo, which hosts the most massive stars in our simulation, has a shallower IMF slope of α ≈ 1, while later-forming minihalos produce lower maximum stellar masses in lower-density parent clouds.

Type

Preprint

Publication

Submitted to Physical Review D