Molecular Gas and the Star Formation Process on Cloud Scales in Nearby Galaxies

Abstract: Observations that resolve nearby galaxies into individual regions across multiple phases of the gas-star formation-feedback ``matter cycle'' have provided a sharp new view of molecular clouds, star formation efficiencies, timescales for region evolution, and stellar feedback. We synthesize these results, cover aspects relevant to the interpretation of observables, and conclude that: (1) The observed cloud-scale molecular gas surface density, line width, and internal pressure all reflect the large-scale galactic environment while also appearing mostly consistent with properties of a turbulent medium strongly affected by self-gravity. (2) Cloud-scale data allow for statistical inference of both evolutionary and physical timescales. These suggest that clouds collapse on timescale of order the free-fall or turbulent crossing time ($\sim 10{-}30$~Myr) followed by the formation of massive stars and subsequent rapid ($\lesssim$ 5 Myr) gas clearing. The star formation efficiency per free-fall time is well determined over thousands of regions to be $\epsilon_{\rm ff}\approx 0.5_{-0.3}^{+0.7}\%$. (3) The role of stellar feedback is now measured using multiple observational approaches. The net momentum yield is constrained by the requirement to support the vertical weight of the galaxy disk. Meanwhile, the short gas clearing timescales suggest a large role for pre-supernova feedback in cloud disruption. This leaves the supernovae free to exert a large influence on the larger scale galaxy, including driving turbulence, launching galactic-scale winds, and carving superbubbles.