Quantifying the informativity of emission lines to infer physical conditions in giant molecular clouds

Context. Observations of ionic, atomic, or molecular lines are performed to improve our understanding of the interstellar medium (ISM). However, the potential of a line to constrain the physical conditions of the ISM is difficult to assess quantitatively, because of the complexity of the ISM physics. The situation is even more complex when trying to assess which combinations of lines are the most useful. Therefore, observation campaigns usually try to observe as many lines as possible for as much time as possible. Aims. We have searched for a quantitative statistical criterion to evaluate the full constraining power of a (combination of) tracer(s) with respect to physical conditions. Our goal with such a criterion is twofold. First, we want to improve our understanding of the statistical relationships between ISM tracers and physical conditions. Secondly, by exploiting this criterion, we aim to propose a method that helps observers to make their observation proposals; for example, by choosing to observe the lines with the highest constraining power given limited resources and time. Methods. We propose an approach based on information theory, in particular the concepts of conditional differential entropy and mutual information. The best (combination of) tracer(s) is obtained by comparing the mutual information between a physical parameter and different sets of lines. The presented analysis is independent of the choice of the estimation algorithm (e.g., neural network or χ 2 minimization). We applied this method to simulations of radio molecular lines emitted by a photodissociation region similar to the Horsehead Nebula. In this simulated data, we considered the noise properties of a state-of-the-art single dish telescope such as the IRAM 30m telescope. We searched for the best lines to constrain the visual extinction, A V tot , or the ultraviolet illumination field, G 0 . We ran this search for different gas regimes, namely translucent gas, filamentary gas, and dense cores. Results. The most informative lines change with the physical regime (e.g., cloud extinction). However, the determination of the optimal (combination of) line(s) to constrain a physical parameter such as the visual extinction depends not only on the radiative transfer of the lines and chemistry of the associated species, but also on the achieved mean signal-to-noise ratio. The short integration time of the CO isotopologue J = 1 − 0 lines already yields much information on the total column density for a large range of ( A V tot , G 0 ) space. The best set of lines to constrain the visual extinction does not necessarily combine the most informative individual lines. Precise constraints on the radiation field are more difficult to achieve with molecular lines. They require spectral lines emitted at the cloud surface (e.g., [CII] and [CI] lines). Conclusions. This approach allows one to better explore the knowledge provided by ISM codes, and to guide future observation campaigns.

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Publication Details

Journal
Astronomy and Astrophysics
Published
2024-09-30
DOI
https://doi.org/10.1051/0004-6361/202451588
Citations
7
Primary Topic
Astrophysics and Star Formation Studies
Type
article
Field-Weighted Citation Impact
2.32

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article

Quantifying the informativity of emission lines to infer physical conditions in giant molecular clouds

Émeric Bron, Maryvonne Gérin, Antoine Zakardjian, Simon Coudé et al.
7 citations
Astronomy and Astrophysics
Astrophysics and Star Formation Studies
2.32
article

Quantifying the informativity of emission lines to infer physical conditions in giant molecular clouds

Émeric Bron, Maryvonne Gérin, Antoine Zakardjian, Simon Coudé, P. Gratier, J. Pety, J. R. Goicoechea, Karine Demyk, Antoine Roueff, Pierre Palud, Miriam G. Santa-Maria, Annie Hughes, D. C. Lis, A. Sievers, E. Roueff, Pierre-Antoine Thouvenin, N. Peretto, David Languignon, Viviana V. Guzmán, Victor de Souza Magalhães, H. S. Liszt, J. Le Bourlot, François Levrier, Lucas Einig, Pierre Chainais, Jan H. Orkisz, Léontine Ségal, Franck Le Petit, Helena Mazurek, Jocelyn Chanussot, Ivana Bešlić, Sébastien Bardeau
article en
7 citations

Abstract

Context. Observations of ionic, atomic, or molecular lines are performed to improve our understanding of the interstellar medium (ISM). However, the potential of a line to constrain the physical conditions of the ISM is difficult to assess quantitatively, because of the complexity of the ISM physics. The situation is even more complex when trying to assess which combinations of lines are the most useful. Therefore, observation campaigns usually try to observe as many lines as possible for as much time as possible. Aims. We have searched for a quantitative statistical criterion to evaluate the full constraining power of a (combination of) tracer(s) with respect to physical conditions. Our goal with such a criterion is twofold. First, we want to improve our understanding of the statistical relationships between ISM tracers and physical conditions. Secondly, by exploiting this criterion, we aim to propose a method that helps observers to make their observation proposals; for example, by choosing to observe the lines with the highest constraining power given limited resources and time. Methods. We propose an approach based on information theory, in particular the concepts of conditional differential entropy and mutual information. The best (combination of) tracer(s) is obtained by comparing the mutual information between a physical parameter and different sets of lines. The presented analysis is independent of the choice of the estimation algorithm (e.g., neural network or χ 2 minimization). We applied this method to simulations of radio molecular lines emitted by a photodissociation region similar to the Horsehead Nebula. In this simulated data, we considered the noise properties of a state-of-the-art single dish telescope such as the IRAM 30m telescope. We searched for the best lines to constrain the visual extinction, A V tot , or the ultraviolet illumination field, G 0 . We ran this search for different gas regimes, namely translucent gas, filamentary gas, and dense cores. Results. The most informative lines change with the physical regime (e.g., cloud extinction). However, the determination of the optimal (combination of) line(s) to constrain a physical parameter such as the visual extinction depends not only on the radiative transfer of the lines and chemistry of the associated species, but also on the achieved mean signal-to-noise ratio. The short integration time of the CO isotopologue J = 1 − 0 lines already yields much information on the total column density for a large range of ( A V tot , G 0 ) space. The best set of lines to constrain the visual extinction does not necessarily combine the most informative individual lines. Precise constraints on the radiation field are more difficult to achieve with molecular lines. They require spectral lines emitted at the cloud surface (e.g., [CII] and [CI] lines). Conclusions. This approach allows one to better explore the knowledge provided by ISM codes, and to guide future observation campaigns.

Astronomy and AstrophysicsVol. 691
Institut polytechnique de Grenoble (FR), California Institute of Technology (US), Centre National de la Recherche Scientifique (FR), Jet Propulsion Laboratory (US), Université Toulouse III - Paul Sabatier (FR), Université de Toulon (FR), Université de Bordeaux (FR), Pontificia Universidad Católica de Chile (CL), Université Paris Cité (FR), Université de Lille (FR), National Radio Astronomy Observatory (US), Université Paris Sciences et Lettres (FR), Laboratoire AstroParticule et Cosmologie (FR), Worcester State University (US), École Normale Supérieure - PSL (FR), University of Florida (US), Sorbonne Université (FR), Sorbonne Paris Cité (FR), Instituto de Física Fundamental (ES), Institut de Radioastronomie Millimétrique (FR), Laboratoire d'Astrophysique de Bordeaux (FR), Centre Inria de l'Université Grenoble Alpes (FR), Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères (FR), Institut des Matériaux, de Microélectronique et des Nanosciences de Provence (FR), Centre de Recherche en Informatique (FR), Instituto de Ciencia de Materiales de Madrid (ES), Center for Astrophysics Harvard & Smithsonian (US), GIPSA-Lab (FR), Laboratoire de Physique de l'Ecole Normale Supérieure (FR), CY Cergy Paris Université (FR), Ausonius Institut de Recherche sur l'Antiquité et le Moyen age (FR), Institut de Recherche en Astrophysique et Planétologie (FR), Chalmers University of Technology (SE), École Centrale de Lille (FR), Cardiff University (GB), Université Grenoble Alpes (FR)
National Science Foundation, National Aeronautics and Space Administration, California Institute of Technology, Agence Nationale de la Recherche, Centre National d’Etudes Spatiales, Centre National de la Recherche Scientifique, Jet Propulsion Laboratory
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