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We have observed far-infrared fine structure lines of N, O, and S in five H~II regions in the far outer Galaxy ($R > 15$~kpc) using the Kuiper Airborne Observatory (KAO). These sources were selected from a radio continuum search we made with the VLA and Australia Telescope Compact Array. The far-infrared line observations, combined with the radio continuum fluxes from these regions, allow us to determine the abundances relative to hydrogen of these three elements in an extreme part of the Galaxy. These observations have extended, by almost a factor of 2, the range of galactocentric radius (R) over which abundances have been determined using this technique.
Earlier studies of abundances in the Milky Way have indicated that there is a gradient in heavy element abundances between the Galactic center and the Solar circle, but the distribution in the outer Galaxy has been less certain (Shaver et al. 1983, MNRAS, 204, 53; Fich \& Silkey 1991, ApJ, 366, 107); Simpson et al. 1995, ApJ, submitted), with one study indicating that the N abundance flattens out beyond the Solar circle (Fich \& Silkey 1991). Our results seem to contradict this finding. Our abundance measurements are consistent with those found by Fich and Silkey for individual outer Galaxy HII regions. However, when they are combined with measurements made of inner Galaxy H~II regions using the identical instruments and analysis techniques, our data suggest that the abundances in all three elements, N, O, and S, continue to fall in the outer Galaxy. For example, our results indicate that the abundance of N is 2--3 times lower in the outer Galaxy than at the Solar neighborhood, and is 10 times lower than in the inner Galaxy.
This result, in addition to having important implications for models of Galactic chemical structure and evolution (see, e.g., Wilson and Matteucci 1992, A\&AR, 4, 1), also confirms that the outer Galaxy is an ideal place to study a region of star-formation very different from the Solar neighborhood, but, unlike in other galaxies, close enough to allow detailed studies of individual star-forming regions.
