Extinction and Metal Abundances in the Outer Galaxy

Several surveys for H II regions in the outer Galaxy have failed to reveal the large number of "new" (i.e. previously unknown) H II regions expected. Previous catalogs of H II regions were primarily derived from optical surveys and it was thought that extinction probably hid many more H II regions. One possible explanation for this lack of H II regions is that the extinction throughout the outer Galaxy may be substantially less than previously thought, as low as 0.4 mag/kpc on average. This is consistent with the extinction being dependent on the dust/gas ratio, the dust/gas ratio depending on the metallicity, and the metallicity continuing to decrease with galactocentric distance at the same rate as the metal abundance gradient observed near the Sun.