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

Abstract

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 _⊙ yr^-1 of HVC inflow versus ≈0.20 ± 0.10 M _⊙ yr^-1 of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.