Spiral molecular front in galaxies: Quick transition from atomic to molecular hydrogen in spiral arms

We derived a two-dimensional map of the molecular fraction, f(mol) (ratio of the molecular gas density to that of total gas), in the spiral galaxy M 51, and examined the behavior of molecular fronts (MF), where MF denotes the place where f(mol) changes drastically from nearly zero to unity, and vice versa. We showed that the MF phenomenon is not only radial, but also occurs in the azimuth direction through the spiral arms, and f(mol) changes rapidly in the arm-to-interarm transition regions. The existence of the azimuthal MF indicates that the atomic gas (H 1) is quickly transformed to molecular gas (H-2) during passage through spiral arms. We performed a numerical simulation of MF based on an H1-to-H-2 phase-transition theory, and reproduced the observations. We estimated the azimuthal scale length of the transition to be less than 200 pc, corresponding to a time scale of similar to 2 Myr for H-2 gas formation. The azimuthal width of a molecular arm was estimated to be at most 2.5 kpc. where the gas can remain in the molecular phase for about 25 Myr.