Halo Hydrogen

Neutral hydrogen is perhaps the most readily observed type of gas in the Universe. Milky Way Halo group member David French led a survey using FUSE UV spectroscopy to study neutral hydrogen (H I) in and around the Galaxy in absorption high-order Lyman series lines. His project used 25 lines of sight to luminous background galaxies spread across the sky to look at the spatial distribution of the gas across the sky. This provided a metal-independent view of the diffuse baryons in the Galactic halo, and yielded the column density distribution function (CDDF) down to low H I column density (see figure below). This work is published in the Astrophysical Journal in French et al. (2021). In ongoing work, we have a funded Hubble program to explore how beam-smearing impacts H I measurements taken with 21 cm radio telescopes, and how this impacts metallicity measurements for halo clouds.