Revisiting the Vertical Distribution of H i Absorbing Clouds in the Solar Neighborhood

The vertical distribution of cold neutral hydrogen (H i ) clouds is a constraint on models of the structure, dynamics, and hydrostatic balance of the interstellar medium. In 1978, Crovisier pioneered a method to infer the vertical distribution of H i absorbing clouds in the solar neighborhood. Using data from the Nançay 21 cm absorption survey, Crovisier determined the mean vertical displacement of cold H i clouds, 〈∣ z ∣〉. We revisit that author’s analysis and explore the consequences of truncating the H i absorption sample in Galactic latitude. For any nonzero latitude limit, we find that the quantity inferred by Crovisier is not the mean vertical displacement but rather a ratio involving higher moments of the vertical distribution. The resultant distribution scale heights are thus ∼1.5 to ∼3 times smaller than previously determined. In light of this discovery, we develop a Bayesian Monte Carlo Markov Chain method to infer the vertical distribution of H i absorbing clouds. We fit our model to the original Nançay data and find a vertical distribution moment ratio 〈∣ z ∣ ^3 〉/〈∣ z ∣ ^2 〉 = 97 ± 15 pc, which corresponds to a Gaussian scale height σ _z = 61 ± 9 pc, an exponential scale height λ _z = 32 ± 5 pc, and a rectangular half-width W _z _,1/2 = 129 ± 20 pc. Consistent with recent simulations, the vertical scale height of cold H i clouds appears to remain constant between the inner Galaxy and the Galactocentric distance of the solar neighborhood. Local fluctuations might explain the large-scale height observed at the same Galactocentric distance on the far side of the Galaxy.