Molecular hydrogen is thought to form from atomic hydrogen on the surface of dust grains in the interstellar medium, and to be dissociated primarily by ultraviolet radiation from hot stars1. This process has been modelled on the 1 pc length scale for the case of dense interstellar clouds2 and near newly formed ionizing stars3. Observational evidence that molecular hydrogen is indeed dissociated in shells around young OB stars in the Galaxy has been presented recently4-6. We suggest here that the same dissociation process occurs on the kpc scale in active star-forming regions of some galaxies, and that this dissociation may strongly affect the observed morphology of atomic hydrogen in spiral arms.
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