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The diffuse, ionized gas at the interface of the Milky Way’s interstellar medium (ISM) and circumgalactic medium is a reservoir of future star-forming fuel and of metals generated by stars in the Galaxy. Metals in this gas are tracers of the feedback processes that drive the Galactic fountain. However, accurate measurements of the metallicity in this region are challenging due to the uncertain ionization correction, and blending with the Milky Way ISM. In this talk, I will present the metallicity measurements of ionized gas clouds along the sight lines of 8 halo globular clusters (GCs) at galactic heights of z 10 kpc. We use the archival HST/COS and HST/STIS spectra of the UV bright stars in the GCs, and address the blending by fitting Voigt profiles to all absorption components. The total hydrogen column is strictly constrained by pulsar dispersion measurement for the same GCs. I will discuss the Galactic Fountain mechanism as a probable origin of infalling gas clouds based on the decreasing metallicity with decreasing infall time. Moreover, a large scatter in metallicity is also found, and I will discuss its potential impact on the metallicity distribution of the ISM. |
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