2024A&A...687A.293Q

Spiral arms, as those of our own Milky Way, are some of the most spectacular features in disc galaxies. It has been argued that star formation should proceed more efficiently in spiral arms as a result of gas compression. Yet, observational studies have so far yielded contradictory results. Here, we examine arm/interarm surface density contrasts at ∼100 pc resolution in 28 spiral galaxies from the PHANGS survey. We find that the arm/interarm contrast in stellar mass surface density (Σ_⋆) is very modest, typically a few tens of percent. This is much smaller than the contrasts measured for molecular gas (Σ_mol) or star formation rate (Σ_SFR) surface density, which typically reach a factor of ∼2 - 3. However, Σ_mol and Σ_SFR contrasts show a significant correlation with the enhancement in Σ_⋆, suggesting that the small stellar contrast largely dictates the stronger accumulation of gas and star formation. All these contrasts increase for grand-design spirals compared to multi-armed and flocculent systems (and for galaxies with high stellar mass). The median star formation efficiency (SFE) of the molecular gas is 16% higher in spiral arms than in interarm regions, with a large scatter, and the contrast increases significantly (median SFE contrast 2.34) for regions of particularly enhanced stellar contrast (Σ_⋆ contrast > 1.97). The molecular-to-atomic gas ratio (Σ_mol/Σ_atom) is higher in spiral arms, pointing to a transformation of atomic to molecular gas. As a consequence, the total gas contrast (Σ_mol + Σ_atom) slightly drops compared to Σ_mol (median 4% lower, working at ∼kpc resolution), while the SFE contrast increases when we include atomic gas (median 8% higher than for Σ_mol). The contrasts show important fluctuations with galactocentric radius. We confirm that our results are robust against a number of effects, such as spiral mask width, tracers, resolution, and binning. In conclusion, the boost in the SFE of molecular gas in spiral arms is generally modest or absent, except for locations with exceptionally large stellar contrasts.