A new preprint (Sultan et al., arXiv:2604.14273, not yet peer-reviewed) uses the high-resolution FIRE-2 cosmological zoom-in hydrodynamical simulations to track how gas temperature shapes angular momentum delivery from dark matter halos to galactic disks. The study focuses on halos around the critical ∼10^12 solar mass scale, where multiple transitions occur: virialization of the inner circumgalactic medium (CGM), a shift from bursty to steady star formation, and the emergence of thin stellar disks. Methodology involves Lagrangian particle tracking of gas temperature, specific angular momentum, and orbital circularity across dozens of simulated galaxies, from high redshift to z=0. While FIRE implements detailed stellar feedback, supernovae, radiation pressure, and black hole physics on parsec scales, limitations include modest sample sizes typical of zoom-ins (roughly a dozen halos spanning the key mass range) and reliance on sub-grid models that may not fully capture all CGM physics.

The core result is clear in plain terms: below the critical halo mass, almost all gas flows in cold (T < 10^5 K) and carries higher specific angular momentum than the dark matter. Yet this high-spin gas does not settle into a rotating disk. Instead, it collapses into stars within fewer than five galaxy free-fall times, producing the irregular, bursty galaxies we see in the dwarf regime. Above the mass threshold, the inner CGM virializes, hot inflows (T > 10^5 K) dominate even at high redshift, and the gas circularizes at galactic radii before cooling. These systems allow gas to linger for up to 25 free-fall times, building rotationally supported thin disks and producing steady star formation.

This work goes well beyond earlier cold-stream literature. Classic papers such as Kereš et al. (2005, arXiv:astro-ph/0504041) first showed that cold accretion should dominate at high redshift, but lacked the resolution and feedback to follow angular momentum from CGM to star-forming gas. Hopkins et al. (2018, arXiv:1701.07027) established the FIRE-2 framework and documented the bursty-to-steady transition, yet stopped short of systematically linking hot-gas circularization to angular-momentum conservation across scales. The new preprint fills that gap, revealing that the long-standing 'angular momentum catastrophe' in galaxy formation theory is largely a question of thermal state: cold streams deliver spin too early and too violently for ordered rotation, while hot halos permit torques and gradual cooling at the radii where disks actually form.

What most coverage has missed is the persistence of hot inflows even at z > 2 in massive halos. This challenges purely cold-stream-driven models of high-redshift galaxy assembly and aligns with emerging JWST data showing surprisingly mature disks at early times. The pattern also explains why Milky Way-mass galaxies host thin disks while lower-mass systems remain puffy and irregular. In essence, the thermal history of the CGM acts as a gatekeeper for angular momentum transport, turning an unsolved theoretical headache into a predictable outcome of halo virialization.

By synthesizing these simulation results with both older theoretical work and newest observations, a coherent picture emerges: galaxy morphology, kinematics, and star-formation regularity are not simply set by total mass, but by whether accreted gas has time and space to respect its angular momentum before turning into stars. This FIRE analysis provides one of the clearest theoretical bridges yet between small-scale feedback physics and large-scale galaxy demographics.