If, throughout a GMC, the ambipolar diffusion time
tAD were to instantaneously
drop to a value less than the local free-fall time, the bomb would
certainly ignite.
Since tAD depends directly
on the strength of the magnetic field, one might imagine a
scenario by which the B-field inside the GMC decreases during
a galaxy-galaxy interaction ... but you would be hard-pressed to
find anyone who would buy such a scenario!
But it recently has dawned on us that,
via the ionization rate zCR,
the ambipolar diffusion time also depends on the flux of cosmic
rays in the vicinity of the GMC, and
if any GMC from the disk of either galaxy is ballistically displaced
to a location well outside the galaxy's originally organized disk,
the GMC should explode in a burst of star formation.
Alternatively, when the ordered nature of either galaxy disk
becomes sufficiently tidally disturbed during a galaxy-galaxy
interaction,
the large-scale field that has been responsible for
trapping the cosmic rays in a magnetic bottle near the galaxy disk
may "open up," thereby permitting the cosmic rays to escape and
igniting many of the GMCs that have remained with the disk.