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.
The Outer Galaxy
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