A steep mass transition for bar-driven interstellar medium structuring revealed by PHANGS-JWST

Galactic bars are thought to play a critical role in the secular evolution of their hosts by, for example, reorganising the interstellar medium (ISM). We used a sample of 57 star-forming disc galaxies observed with JWST at 3 and 7.7 µm to probe how the spatial distribution of polycyclic aromatic hydrocarbon (PAH) emission as a structural marker of the cold ISM might depend on galaxy stellar mass and the presence of a bar. We find evidence for a ’watershed’ at a stellar mass of 1010 M⊙, marking a fundamental transition in the bar-driven distribution of PAH emission. This confirms trends previously predicted by numerical simulations and observed via ionised gas or UV light. While lower-mass galaxies exhibit a disordered and clumpy distribution of PAH emission regardless of bar presence, higher-mass barred hosts display well-structured dynamical features traced by PAH emission with significant gas reservoirs (e.g., discs and rings) within the central 15% of the bar radius (Rb). Furthermore, we observed a systematic depletion of PAH emission within the [0.2–0.8] Rb range in barred systems with stellar masses above 1010 M⊙. Such central discs, rings, and associated radial dips (‘bar deserts’) appear to be a mass-dependent phenomenon as they are ubiquitous in massive galaxies but mostly absent in their lower-mass counterparts. In contrast to the structured features in massive hosts, the disorganised ISM in lower-mass galaxies masks the commonly observed bar-driven signatures. This suggests that tracer selection and dust obscuration may significantly bias observed bar fractions. Our study underlines the existence of two regimes of secular evolution, with different impacts and observability of bar-driven processes, and it reaffirms the role of bars as the primary drivers of rapid secular evolution in galaxies above 1010 M⊙, while the impact of bars is significantly reduced or delayed below this threshold. Our results further underscore the need to critically account for these processes when modelling galaxy evolution in cosmological simulations.