The lifecycle of molecular clouds in nearby star-forming disc galaxies
  It remains a major challenge to derive a theory of cloud-scale ($\lesssim100$
pc) star formation and feedback, describing how galaxies convert gas into stars
as a function of the galactic environment. Progress has been hampered by a lack
of robust empirical constraints on the giant molecular cloud (GMC) lifecycle.
We address this problem by systematically applying a new statistical method for
measuring the evolutionary timeline of the GMC lifecycle, star formation, and
feedback to a sample of nine nearby disc galaxies, observed as part of the
PHANGS-ALMA survey. We measure the spatially-resolved ($\sim100$ pc)
CO-to-H$\alpha$ flux ratio and find a universal de-correlation between
molecular gas and young stars on GMC scales, allowing us to quantify the
underlying evolutionary timeline. GMC lifetimes are short, typically 10-30 Myr,
and exhibit environmental variation, between and within galaxies. At kpc-scale
molecular gas surface densities $\Sigma_{\rm
H_2}\geqslant8$M$_{\odot}$pc$^{-2}$, the GMC lifetime correlates with
time-scales for galactic dynamical processes, whereas at $\Sigma_{\rm
H_2}\leqslant8$M$_{\odot}$pc$^{-2}$ GMCs decouple from galactic dynamics and
live for an internal dynamical time-scale. After a long inert phase without
massive star formation traced by H$\alpha$ (75-90% of the cloud lifetime), GMCs
disperse within just 1-5 Myr once massive stars emerge. The dispersal is most
likely due to early stellar feedback, causing GMCs to achieve integrated star
formation efficiencies of 4-10% These results show that galactic star formation
is governed by cloud-scale, environmentally-dependent, dynamical processes
driving rapid evolutionary cycling. GMCs and HII regions are the fundamental
units undergoing these lifecycles, with mean separations of 100-300 pc in
star-forming discs. Future work should characterise the multi-scale physics and
mass flows driving these lifecycles.
