Molecular Gas in the Inner 3.2 Kiloparsecs of NGC 2403: Star Formation at Subcritical Gas Surface Densities

Abstract

We present a fully sampled map of the inner 3.2 kpc of the nearby spiral galaxy NGC 2403 in the CO J=1-0 line. These data emphasize the relatively small contribution of molecular hydrogen to the cold gas content of this galaxy, and confirm that the gas surface densities in the inner 2.8 kpc of NGC 2403 lie below the critical surface density for star formation under the theory proposed by Kennicutt (1989). Since star formation is occurring throughout the inner disk, the simple dynamical model used by Kennicutt cannot be the only important process regulating star formation in galaxies. We suggest that stochastic star formation processes are responsible for the star formation seen in these regions, and thus that supercritical gas densities may not be a necessary condition for star formation in the inner regions of galactic disks.