Revisiting the Vertical Distribution of HI Absorbing Clouds in the Solar Neighborhood
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arXiv
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| Formato: | Preprint |
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2024
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| _version_ | 1866917624655380480 |
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| author | Wenger, Trey V. Rybarczyk, Daniel R. Stanimirović, Snežana |
| author_facet | Wenger, Trey V. Rybarczyk, Daniel R. Stanimirović, Snežana |
| contents | 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, $\langle|z|\rangle$. 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 ${\sim}1.5$ to ${\sim}3$ 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 $\langle|z|^3\rangle/\langle|z|^2\rangle = 97 \pm 15\,\text{pc}$, which corresponds to a Gaussian scale height $σ_z = 61 \pm 9\,\text{pc}$, an exponential scale height $λ_z = 32 \pm 5\,\text{pc}$, and a rectangular half-width $W_{z, 1/2} = 129 \pm 20\,\text{pc}$. 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2403_18981 |
| institution | arXiv |
| publishDate | 2024 |
| record_format | arxiv |
| spellingShingle | Revisiting the Vertical Distribution of HI Absorbing Clouds in the Solar Neighborhood Wenger, Trey V. Rybarczyk, Daniel R. Stanimirović, Snežana Astrophysics of Galaxies 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, $\langle|z|\rangle$. 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 ${\sim}1.5$ to ${\sim}3$ 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 $\langle|z|^3\rangle/\langle|z|^2\rangle = 97 \pm 15\,\text{pc}$, which corresponds to a Gaussian scale height $σ_z = 61 \pm 9\,\text{pc}$, an exponential scale height $λ_z = 32 \pm 5\,\text{pc}$, and a rectangular half-width $W_{z, 1/2} = 129 \pm 20\,\text{pc}$. 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. |
| title | Revisiting the Vertical Distribution of HI Absorbing Clouds in the Solar Neighborhood |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2403.18981 |