Star formation in Galactic flows
  We investigate the triggering of star formation in clouds that form in
Galactic scale flows as the ISM passes through spiral shocks. We use the
Lagrangian nature of SPH simulations to trace how the star forming gas is
gathered into self-gravitating cores that collapse to form stars. Large scale
flows that arise due to Galactic dynamics create shocks of order 30 km/s that
compress the gas and form dense clouds $(n> $several $\times 10^2$ cm$^{-3}$)
in which self-gravity becomes relevant. These large-scale flows are necessary
for creating the dense physical conditions for gravitational collapse and star
formation. Local gravitational collapse requires densities in excess of
$n>10^3$ cm$^{-3}$ which occur on size scales of $\approx 1$ pc for low-mass
star forming regions ($M<100 M_{\odot}$), and up to sizes approaching 10 pc for
higher-mass regions ($M>10^3 M_{\odot}$). Star formation in the 250 pc region
lasts throughout the 5 Myr timescale of the simulation with a star formation
rate of $\approx 10^{-1} M_{\odot}$ yr$^{-1}$ kpc$^{-2}$. In the absence of
feedback, the efficiency of the star formation per free-fall time varies from
our assumed 100 % at our sink accretion radius to values of $< 10^{-3}$ at low
densities.
