The Chemical Enrichment of the Diffuse Gas in the Outer Galaxy and the Abundance Gradient of the Milky Way

摘要

Using data from HST/STIS (Hubble Space Telescope / Space Telescope ImagingSpectrograph) and FUSE (Far Ultraviolet Spectroscopic Explorer) toward two QSOs, H1821+643and HS0624+6907, we find that the overall metallicity of the Galactic "Outer Arm" is Z = 0.3 —0.5Z with underabundant nitrogen and little depletion by dust. The results are consistent withthose based on H II region measurements in the outer galaxy and provide additional constrainson models of the Galactic abundance gradient and Milky Way (MW) chemical evolution. Thelower metallicity observed in the outer galaxy is consistent with abundance patterns observedin higher redshift damped Lyn absorbers (DLAs) extrapolated to z=0. The slow metallicityevolution of DLAs could be due to the larger cross sections of the outer regions of galaxiescombined with the observed metallicity gradients.