Determine the physical cause of the specific-angular-momentum transition

Determine the single physical cause, or combination of causes, responsible for the sudden increase in stellar specific angular momentum experienced by higher-mass galaxies in the IllustrisTNG simulations, including the possible roles of bursty star formation, deepening gravitational potentials, and changes in gas-accretion mode.

Background

The IllustrisTNG galaxy histories show that some galaxies undergo a rapid increase in stellar specific angular momentum after initially losing angular momentum, followed by the formation of a stable rotating disk. The paper examines several possible explanations, including mergers, numerical resolution, bursty star formation, changes in the gravitational potential, and gas-accretion physics.

Although the analysis rules out some explanations as sole causes and identifies several processes that may contribute, it does not establish a unique mechanism. The unresolved problem is therefore to determine which physical processes drive the transition and how they jointly produce disk formation and the associated increase in specific angular momentum.

References

Assuming a physical origin, we investigated many potential mechanisms for this sudden change in $j_{*}$ experienced by the higher-mass galaxies, but were unable to conclusively determine a single cause of this change.

— The H$α$ specific angular momentum of dwarf galaxies  (2608.16089 - Deeley et al., 17 Aug 2026) in Section 5.2, “The cause of the jump in $j_{*}$”; Section 7, “Summary and Conclusion”

It remains possible that this transition in $j_{*}$ corresponds to a resolution effect, for example the crossing of a threshold in the number of particles within the galaxy resulting in an alteration of internal dynamics.

— The H$α$ specific angular momentum of dwarf galaxies  (2608.16089 - Deeley et al., 17 Aug 2026) in Section 5.1, “Evolution of $j_{*}$,” subsection “Transition in $j_{*}$ and disk formation”