A New Ignition Mechanism

  • 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.

    How might this be accomplished?

  • 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,

    1. via the ionization rate zCR, the ambipolar diffusion time also depends on the flux of cosmic rays in the vicinity of the GMC, and

    2. from the "leaky box" model of cosmic-ray propagation, we know that the flux of cosmic rays is strongly concentrated toward the disk of the galaxy.

  • Hence, during a galaxy-galaxy interaction

    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.

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