Direct versus indirect role of gas in bar formation

Determine whether gas affects the formation of stellar bars directly through angular momentum exchange between the gaseous component and the stellar bar, or indirectly via the creation of a central mass concentration (CMC) produced by bar-driven gas inflows.

Background

The role of gas in the onset and evolution of stellar bars has long been debated. Early simulation studies suggested that higher gas fractions can weaken or even dissolve bars, attributing this to gas inflows that build compact central mass concentrations (CMCs) and/or to angular momentum transfer between gas and bar. Subsequent work proposed that bar weakening correlates more with bar buckling driven by the CMC rather than with direct gas–bar angular momentum exchange.

Given these differing interpretations, a key unresolved question is whether gas acts as a direct dynamical participant in bar formation by exchanging angular momentum with the stellar bar, or whether its influence is predominantly indirect by forming a CMC that modifies the inner potential and stabilizes the disc against bar growth.

References

Therefore, while gas appears to play a role in bar formation, it is unclear whether this occurs directly' due to the exchange of angular momentum between the gas and the bar itself, or whether gas has anindirect' effect on bar formation, for example through the formation of a CMC which forms when gas is pushed to the centre by the bar.

— Bar formation and evolution in the cosmological context: Inputs from the Auriga simulations (2406.09453 - Fragkoudi et al., 12 Jun 2024) in Introduction