AR 15795 (Archival Research)

Thu Jul 18 14:08:44 GMT 2024

Principal Investigator: David French
PI Institution: Space Telescope Science Institute
Investigators (xml)

Title: Self-Consistent Metallicities in the Galactic Halo
Cycle: 27

We propose a new technique for determining gas-phase metallicity measurements in the Galactic halo that is independent of beam-smearing effects. Beam smearing effects result from combining radio 21 cm HI emission measurements with UV metal-absorption measurements. This can lead to systematic errors in metallicity estimates that prevent accurate comparisons to theoretical predictions and extragalactic measurements. Our new technique instead uses Far Ultraviolet Spectroscopic Explorer (FUSE) data covering the full Lyman absorption series to measure HI without relying on 21 cm observations. Our pilot study finds systematically lower HI columns from UV measurements than from 21 cm measurement. When combining with HST measurements of metals, the preliminary results indicate that the metallicity of Galactic halo clouds could be underestimated by at least a factor of 2. Here we propose to confirm these potentially important results via an analysis of 41 archival HST COS or STIS sightlines with complementary FUSE and 21 cm radio data. We propose measurements of OI 1302 and SII 1259, 1253, 1250 absorption from which we will measure the oxygen and sulfur abundances in IVCs and HVCs with both UV and radio HI measurements. With this sample we will quantify the reliability of radio-based metallicities. If confirmed, the offset between radio-derived and UV-derived metallicities would have significant implications on the origin of HVCs and IVCs and on how these local structures relate to extragalactic circumgalactic absorbers.