Extinction and metal abundances in the outer galaxy

Abstract

Several surveys for HII regions in the outer Galaxy have failed to reveal the large number of “new” (i.e. previously unknown HII regions expected. Previous catalogs of HII regions were primarily derived from optical surveys and it was thought that extinction probably hid many more HII regions. One possible explanation for this lack of HII 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.