NGC 628 H II regions

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, which correspond to luminosities of 10^36^ergs/s, or a fraction of the Orion Nebula luminosity.

The most distant object detected is at R~27kpc from the galaxy center. The massive star formation rate, as measured from the azimuthally averaged H{alpha} flux, is consistent with a monotonic decrease as far as R~1.3R_25_ (20kpc), 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 {Sigma}gas<=4M_{sun}_/pc^2^; the index depends strongly on the CO data used.