On the accuracy of H I observations in molecular clouds: more cold H I than thought?
We present a study of the cold atomic hydrogen (H i) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of H i at 21 cm, including H i self-absorption (HISA) and observational effects. We find that H i column dens...
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| Hauptverfasser: | , , , , , , |
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| Dokumenttyp: | Article (Journal) |
| Sprache: | Englisch |
| Veröffentlicht: |
2022 March 7
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| In: |
Monthly notices of the Royal Astronomical Society
Year: 2022, Jahrgang: 512, Heft: 4, Pages: 4765-4784 |
| ISSN: | 1365-2966 |
| DOI: | 10.1093/mnras/stac607 |
| Online-Zugang: | Verlag, lizenzpflichtig, Volltext: https://doi.org/10.1093/mnras/stac607
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| Verfasserangaben: | D. Seifried, H. Beuther, S. Walch, J. Syed, J.D. Soler, P. Girichidis and R. Wünsch |
| Zusammenfassung: | We present a study of the cold atomic hydrogen (H i) content of molecular clouds simulated within the SILCC-Zoom project for solar neighbourhood conditions. We produce synthetic observations of H i at 21 cm, including H i self-absorption (HISA) and observational effects. We find that H i column densities, $N_{\rm H\, \small {\rm I}}$, of ≳1022 cm−2 are frequently reached in molecular clouds with H i temperatures as low as ∼10 K. Hence, HISA observations assuming a fixed H i temperature tend to underestimate the amount of cold H i in molecular clouds by a factor of 3-10 and produce an artificial upper limit of $N_{\rm H\, \small {\rm I}}$ around 1021 cm−2. We thus argue that the cold H i mass in molecular clouds could be a factor of a few higher than previously estimated. Also, $N_{\rm H\, \small {\rm I}}$ PDFs obtained from HISA observations might be subject to observational biases and should be considered with caution. The underestimation of cold H i in HISA observations is due to both the large H i temperature variations and the effect of noise in regions of high optical depth. We find optical depths of cold H i around 1-10, making optical depth corrections essential. We show that the high H i column densities (≳1022 cm−2) can in parts be attributed to the occurrence of up to 10 individual H i-H2 transitions along the line of sight. This is also reflected in the spectra, necessitating Gaussian decomposition algorithms for their in-depth analysis. However, also for a single H i-H2 transition, $N_{\rm H\, \small {\rm I}}$ frequently exceeds 1021 cm−2, challenging one-dimensional, semi-analytical models. This is due to non-equilibrium chemistry effects and the fact that H i-H2 transition regions usually do not possess a one-dimensional geometry. Finally, we show that the H i gas is moderately supersonic with Mach numbers of a few. The corresponding non-thermal velocity dispersion can be determined via HISA observations within a factor of ∼2. |
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| Beschreibung: | Gesehen am 25.05.2022 |
| Beschreibung: | Online Resource |
| ISSN: | 1365-2966 |
| DOI: | 10.1093/mnras/stac607 |