| Abstract | Employing deep Hα narrowband imagery, we identified and measured the fluxes of 137 small H II regions in the extreme outer disk (R>=R25) of the nearly face-on, Sc I galaxy NGC 628. A majority of these H II regions are located in two faint, outer spiral arms visible in H I maps. The faintest H II regions that could be measured have fluxes of only a few times 10-16 ergs cm-2 s-1, which correspond to luminosities of ~1036 ergs s-1, or a fraction of the Orion Nebula luminosity. The most distant object detected is at R~27 kpc from the galaxy center. The massive star formation rate, as measured from the azimuthally averaged Hα flux, is consistent with a monotonic decrease as far as R~1.3R25 (20 kpc), where there is a sharp drop. The Schmidt law for the whole disk of NGC 628 corresponds to a relatively steep power law with n=2.9+/-0.2, but it ``fails'' below Σgas<=4 Msolar pc-2; the index depends strongly on the CO data used. We derived the luminosity function (LF) of 598 H II regions over the whole disk, and we compared the shapes of the cumulative LFs between R>=R25 and R25 the LF of the outer regions is significantly steeper than that of the inner regions. |
