Isolating signatures of major cloud-cloud collisions II: The lifetimes
  of broad bridge features
  We investigate the longevity of broad bridge features in position-velocity
diagrams that appear as a result of cloud-cloud collisions. Broad bridges will
have a finite lifetime due to the action of feedback, conversion of gas into
stars and the timescale of the collision. We make a series of analytic
arguments with which to estimate these lifetimes. Our simple analytic arguments
suggest that for collisions between clouds larger than R~10 pc the lifetime of
the broad bridge is more likely to be determined by the lifetime of the
collision rather than the radiative or wind feedback disruption timescale.
However for smaller clouds feedback becomes much more effective. This is
because the radiative feedback timescale scales with the ionising flux Nly as
R^{7/4}Nly^{-1/4} so a reduction in cloud size requires a relatively large
decrease in ionising photons to maintain a given timescale. We find that our
analytic arguments are consistent with new synthetic observations of numerical
simulations of cloud-cloud collisions (including star formation and radiative
feedback). We also argue that if the number of observable broad bridges remains
~ constant, then the disruption timescale must be roughly equivalent to the
collision rate. If this is the case our analytic arguments also provide
collision rate estimates, which we find are readily consistent with previous
theoretical models at the scales they consider (clouds larger than about 10 pc)
but are much higher for smaller clouds.
