The H i Column Density Distribution of the Galactic Disk and Halo

David M. French, Andrew J. Fox, Bart P. Wakker, Nicolas Lehner, J. Christopher Howk, Blair D. Savage, Philipp Richter, John O'Meara, Sanchayeeta Borthakur, Timothy Heckman, Colin Norman

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(H i) ∼ 1014 cm-2 using measurements of H i Lyman series absorption from the Far Ultraviolet Spectroscopic Explorer. Our results are drawn from an analysis of 25 AGN sight lines spread evenly across the sky with Galactic latitude |b| ≳ 20°. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Lyβ-Lyμ) plus HST measurements of Lyα, we derive the kinematics and column densities of a sample of 152 H i absorption components. While saturation prevents accurate measurements of many components with column densities 17 ≲ log N(H i) ≲ 19, we derive robust measurements at log N(H i) ≲ 17 and log N(H i) ≳ 19. We derive the first ultraviolet H i column density distribution function (CDDF) of the Milky Way, both globally and for low-velocity (ISM), intermediate-velocity clouds (IVCs), and high-velocity clouds (HVCs). We find that IVCs and HVCs show statistically indistinguishable CDDF slopes, with β IVC = -1.01-0.14+0.15 and β HVC = -1.05-0.06+0.07. Overall, the CDDF of the Galactic disk and halo appears shallower than that found by comparable extragalactic surveys, suggesting a relative abundance of high column density gas in the Galactic halo. We derive the sky-covering fractions as a function of H i column density, finding an enhancement of IVC gas in the northern hemisphere compared to the south. We also find evidence for an excess of inflowing H i over outflowing H i, with -0.88 ± 0.40 M o˙ yr-1 of HVC inflow versus ≈0.20 ± 0.10 M o˙ yr-1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.

Original languageEnglish (US)
Article number50
JournalAstrophysical Journal
Issue number1
StatePublished - Dec 10 2021

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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