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 ) ∼ 10 14 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 (Ly β –Ly μ ) plus HST measurements of Ly α , 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.