The HII regions of the extreme outer disk of NGC 628

Employing deep H alpha narrowband imagery, we identified and measured the f luxes of 137 small H II regions in the extreme outer disk (R greater than o r equal to R-25) 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 on ly a few times 10(-16) ergs cm(-2) s(-1), which correspond to luminosities of similar to 10(36) ergs s(-1), or a fraction of the Orion Nebula luminosi ty. The most distant object detected is at R similar to 27 kpc from the gal axy center. The massive star formation rate, as measured from the azimuthal ly averaged H alpha flux, is consistent with a monotonic decrease as far as R approximate to 1.3R(25) (20 kpc), where there is a sharp drop. The Schmi dt law for the whole disk of NGC 628 corresponds to a relatively steep powe r law with n = 2.9 +/- 0.2, but it "fails" below Sigma(gas) less than or eq ual to 4 M-circle dot pc(-2); the index depends strongly on the CO data use d. We derived the luminosity function (LF) of 598 H n regions over the whol e disk, and we compared the shapes of the cumulative LFs between R greater than or equal to R-25 and R < R-25; the LF of the outer regions is signific antly steeper than that of the inner regions.