MOLECULAR GAS IN THE INNER 3.2 KILOPARSECS OF NGC-2403 - STAR-FORMATION AT SUBCRITICAL GAS-SURFACE DENSITIES

We present a fully sampled map of the inner 3.2 kpc of the nearby spir al galaxy NGC 2403 in the CO J = 1-0 line. These data emphasize the re latively small contribution of molecular hydrogen to the cold gas cont ent of this galaxy: in the inner kiloparsec the molecular hydrogen sur face density is 0.6 times that of atomic hydrogen, in contrast with mo st spiral galaxies for which molecular hydrogen is the dominant phase in the inner regions. These data confirm that the gas surface densitie s in the inner 2.8 kpc of NGC 2403 lies below the critical surface den sity for star formation under the theory proposed by Kennicutt (1989). Since star formation is occurring throughout the inner disk, despite the subcritical gas surface densities, this simple dynamical model can not be the only important process regulating star formation in galaxie s. This conclusion is supported by the widespread star formation seen at subcritical gas densities in the inner disks of M33 and some Sa gal axies. We suggest that stochastic star formation processes are respons ible for the star formation seen in these regions and thus that superc ritical gas densities may not be a necessary condition for star format ion in the inner regions of galactic disks.