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

Abstract

Employing deep Ha narrowband imagery, we identiÐed and measured the Ñuxes of 137 small H II regions in the extreme outer disk of the nearly face-on, Sc I galaxy NGC 628. A majority of (R º R 25) 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 Ñuxes of only a few times 10~16 ergs cm~2 s~1, which correspond to lumi-nosities of D1036 ergs s~1, or a fraction of the Orion Nebula luminosity. The most distant object detected is at R D 27 kpc from the galaxy center. The massive star formation rate, as measured from the azimuthally averaged Ha Ñux, is consistent with a monotonic decrease as far as (20 kpc), R B 1.3R 25 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 ÏÏ below pc~2 ; the index depends strongly & gas ¹ 4 M _ 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 and the LF of the outer R º R 25 R \ R 25 ; regions is signiÐcantly steeper than that of the inner regions.