Revisiting the Vertical Distribution of HI Absorbing Clouds in the Solar Neighborhood
arxiv(2024)
摘要
The vertical distribution of cold neutral hydrogen (HI) 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 HI absorbing clouds in the solar neighborhood. Using
data from the Nançay 21-cm absorption survey, they determine the mean
vertical displacement of cold HI clouds, ⟨|z|⟩. We revisit
Crovisier's analysis and explore the consequences of truncating the HI
absorption sample in Galactic latitude. For any non-zero 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 HI 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 HI 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.
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