2000AJ....120.1306L

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 Σ_gas≤4 M_☉.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 R<R₂₅; the LF of the outer regions is significantly steeper than that of the inner regions.: