Gas flows in galactic centre environments: cloud evolution and star
  formation in the Central Molecular Zone
  The Central Molecular Zone (CMZ) is the most extreme star-forming environment
in the Milky Way in terms of gas pressures, densities, temperatures, and
dynamics. It acts as a critical test bed for developing star formation theories
applicable to the (high-redshift-like) conditions under which most stars in the
Universe formed. We present a set of numerical simulations of molecular clouds
orbiting on the 100-pc stream that dominates the molecular gas reservoir of the
CMZ, with the goal of characterising their morphological and kinematic
evolution in response to the external gravitational potential and their
eccentric orbital motion. These simulations capture the evolution of single
clouds in a strong and plausibly dominant background potential. We find that
the evolution of the clouds is closely coupled to the orbital dynamics and
their arrival on the 100-pc stream marks a transformative event in their
lifecycle. The clouds' sizes, aspect ratios, position angles, filamentary
structure, column densities, velocity dispersions, line-of-sight velocity
gradients, angular momenta, and overall kinematic complexity are controlled by
the background potential and their passage through the orbit's pericentre. We
compare these predictions of our simulations to observations of clouds on the
Galactic Centre `dust ridge' and find that the inclusion of galactic dynamics
naturally reproduces a surprisingly wide variety of key observed morphological
and kinematic features. We argue that the accretion of gas clouds onto the
central regions of galaxies, where the rotation curve turns over and the tidal
field becomes fully compressive, is likely to lead to their collapse and
associated star formation. This can generate an evolutionary progression of
cloud collapse with a common zero point. Together, these processes may
naturally give rise to the synchronised starbursts observed in numerous
galactic nuclei.
