The Impact of Shear on Disk Galaxy Star Formation Rates

Determining the physical processes that control galactic-scale star formation rates is essential for an improved understanding of galaxy evolution. The role of orbital shear is currently unclear, with some models expecting reduced star formation rates and efficiencies with increasing shear, e.g., if shear stabilizes gas against gravitational collapse, while others predicting enhanced rates, e.g., if shear-driven collisions between giant molecular clouds trigger star formation. Expanding on the analysis of 16 galaxies by C. Suwannajak et al., we assess the shear dependence of star formation efficiency (SFE) per orbital time (ϵ_orb) in 49 galaxies selected from the PHANGS-ALMA survey. In particular, we test a prediction of the shear-driven giant molecular cloud​​​​​​ (GMC) collision model that ϵ_orb ∝ (1-0.7β), where β ≡ d ln v circ / d ln r , i.e., SFE per orbital time declines with decreasing shear. We fit the function ϵ_orb = ϵ_orb,0(1 − α_CCβ) finding α_CC ≃ 0.76 ± 0.16; an alternative fit with ϵ_orb normalized by the median value in each galaxy yields α CC * = 0.80 ± 0.15 . These results are in good agreement with the prediction of the shear-driven GMC collision theory. We also examine the impact of a galactic bar on ϵ_orb finding a modest decrease in SFE in the presence of a bar, which can be attributed to lower rates of shear in these regions. We discuss the implications of our results for the GMC life cycle and environmental dependence of star formation activity.