Do spiral arms enhance star formation efficiency?

Abstract

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 sim 100\,pc resolution in 28 spiral galaxies from the PHANGS survey. We find that the arm AND interarm contrast in stellar mass surface density ($ is very modest, typically a few tens of percent. This is much smaller than the contrasts measured for molecular gas ($ mol $) or star formation rate ($ SFR $) surface density, which typically reach a factor of $ sim 3$. However, $ mol $ and $ SFR $ contrasts show a significant correlation with the enhancement in $ 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 $16$<!PCT!> 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 ($ contrast $>1.97$). The molecular-to-atomic gas ratio ($ mol atom $) is higher in spiral arms, pointing to a transformation of atomic to molecular gas. As a consequence, the total gas contrast ($ mol atom $) slightly drops compared to $ mol $ (median $4$<!PCT!> lower, working at sim kpc resolution), while the SFE contrast increases when we include atomic gas (median $8$<!PCT!> higher than for $ 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.