The HI Column Density Distribution of the Galactic Disk and Halo

We present a census of neutral gas in the Milky Way disk and halo down to limiting column densities of N(HI) similar to 10(14) cm(-2) using measurements of HI 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 vertical bar b vertical bar greater than or similar to 20 degrees. By simultaneously fitting multi-component Voigt profiles to 11 Lyman series absorption transitions covered by FUSE (Ly beta-Ly mu) plus HST measurements of Ly alpha, we derive the kinematics and column densities of a sample of 152 HI absorption components. While saturation prevents accurate measurements of many components with column densities 17 less than or similar to log N(HI) less than or similar to 19, we derive robust measurements at log N(HI) less than or similar to 17 and log N(HI) greater than or similar to 19. We derive the first ultraviolet H iI 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 beta(IVC) = -1.01(-0.14)(+0.15) beta(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 HI 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 HI, with -0.88 +/- 0.40 M-circle dot yr(-1) of HVC inflow versus approximate to 0.20 +/- 0.10 M-circle dot yr(-1) of HVC outflow, confirming an excess of inflowing HVCs seen in UV metal lines.