Discovery of recent star formation in the extreme outer regions of disk galaxies

We present deep H alpha images of three nearby late-type spiral galaxies (N GC 628, NGC 1058, and NGC 6946), which reveal the presence of H II regions out to, and beyond, two optical radii (defined by the 25th B-band isophote) . The outermost H II regions appear small, faint, and isolated compared to their inner disk counterparts and are distributed in organized spiral arm s tructures, likely associated with underlying H I arms and faint stellar arm s. The relationship between the azimuthally averaged H alpha surface bright ness (proportional to star formation rate per unit area) and the total gas surface density is observed to steepen considerably at low gas surface dens ities. We find that this effect is largely driven by a sharp decrease in th e covering factor of star formation at large radii and not by changes in th e rate at which stars form locally. An azimuthally averaged analysis of the gravitational stability of the disk of NGC 6946 reveals that while the exi stence of star formation in the extreme outer disk is consistent with the T oomre Q instability model, the low rates observed are only compatible with the model when a constant gaseous velocity dispersion is assumed. We sugges t that the observed behavior could also be explained by a model in which th e star formation rate has an intrinsic dependence on the azimuthally averag ed gas volume density, which decreases rapidly in the outer disk due to the vertical flaring of the gas layer.