The H [CSC]ii[/CSC] Regions of the Extreme Outer Disk of NGC 628

Employing deep Hα narrowband imagery, we identified and measured the fluxes of 137 small H II regions in the extreme outer disk ( R ≥ 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 only a few times 10 -16 ergs cm -2 s -1 , which correspond to luminosities of ∼10 36 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 ≈ 1.3 R 25 (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 ≥ R 25 and R < R 25 ; the LF of the outer regions is significantly steeper than that of the inner regions.