2024ApJ...975..167R

The cold neutral medium (CNM) is where neutral atomic hydrogen (H I) is converted into molecular clouds, so the structure and kinematics of the CNM are key drivers of galaxy evolution. Here we provide new constraints on the vertical distribution of the CNM using the recently developed kinematic_scaleheight software package and a large catalog of sensitive H I absorption observations. We estimate the thickness of the CNM in the solar neighborhood to be σ_z ∼ 50-90 pc, assuming a Gaussian vertical distribution. This is a factor of ∼2 smaller than typically assumed, indicating that the thickness of the CNM in the solar neighborhood is similar to that found in the inner Galaxy, consistent with recent simulation results. If we consider only structures with H I optical depths τ > 0.1 or column densities N(H I) > 10^19.5 cm^–2, which recent work suggests are thresholds for molecule formation, we find σ_z ∼ 50 pc. Meanwhile, for structures with τ < 0.1 or column densities N(H I) < 10^19.5 cm^–2, we find σ_z ∼ 120 pc. These thicknesses are similar to those derived for the thin- and thick-disk molecular cloud populations traced by CO emission, possibly suggesting that cold H I and CO are well mixed. Approximately 20% of CNM structures are identified as outliers, with kinematics that are not well explained by Galactic rotation. We show that some of these CNM structures-perhaps representing intermediate-velocity clouds-are associated with the Local Bubble wall. We compare our results to recent observations and simulations, and we discuss their implications for the multiphase structure of the Milky Way's interstellar medium.