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

IF 5.4 2区 物理与天体物理 Q1 ASTRONOMY & ASTROPHYSICS Astronomy & Astrophysics Pub Date : 2024-11-11 DOI:10.1051/0004-6361/202451588
Lucas Einig, Pierre Palud, Antoine Roueff, Jérôme Pety, Emeric Bron, Franck Le Petit, Maryvonne Gerin, Jocelyn Chanussot, Pierre Chainais, Pierre-Antoine Thouvenin, David Languignon, Ivana Bešlić, Simon Coudé, Helena Mazurek, Jan H. Orkisz, Miriam G. Santa-Maria, Léontine Ségal, Antoine Zakardjian, Sébastien Bardeau, Karine Demyk, Victor de Souza Magalhães, Javier R. Goicoechea, Pierre Gratier, Viviana V. Guzmán, Annie Hughes, François Levrier, Jacques Le Bourlot, Dariusz C. Lis, Harvey S. Liszt, Nicolas Peretto, Evelyne Roueff, Albrecht Sievers
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Abstract

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. 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. 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 ( neural network or $ 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 or the ultraviolet illumination field . We ran this search for different gas regimes, namely translucent gas, filamentary gas, and dense cores. The most informative lines change with the physical regime ( 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 ( ) 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 ( and lines). This approach allows one to better explore the knowledge provided by ISM codes, and to guide future observation campaigns.
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量化发射线的信息量以推断巨型分子云的物理条件
对离子、原子或分子线进行观测是为了增进我们对星际介质(ISM)的了解。然而,由于星际介质物理学的复杂性,很难定量评估某条线在约束星际介质物理条件方面的潜力。如果要评估哪种光谱线组合最有用,情况就更加复杂了。因此,观测活动通常会尝试在尽可能多的时间内观测尽可能多的线条。我们一直在寻找一种定量统计标准,以评估示踪线(组合)在物理条件方面的全部约束能力。我们的目标有两个。首先,我们希望加深对 ISM 示踪剂与物理条件之间统计关系的理解。其次,通过利用这一标准,我们希望提出一种方法,帮助观测者提出观测建议;例如,在资源和时间有限的情况下,选择观测约束能力最强的谱线。我们提出的方法基于信息论,特别是条件差分熵和互信息的概念。通过比较物理参数与不同线路组之间的互信息,可以获得最佳(组合)示踪线。所提出的分析方法与估算算法(神经网络或美元最小化)的选择无关。我们将这种方法应用于模拟类似马头星云的光解离区域发射的射电分子线。在模拟数据中,我们考虑了最先进的单碟望远镜(如 IRAM 30 米望远镜)的噪声特性。我们寻找最佳的线条来约束视觉消光或紫外线照明场。我们对不同的气体状态进行了搜索,即半透明气体、丝状气体和致密核心。信息量最大的线条会随着物理机制(云消光)的变化而变化。然而,确定用于约束视觉消光等物理参数的最佳(组合)谱线不仅取决于谱线的辐射传递和相关物种的化学性质,还取决于所达到的平均信噪比。CO 同素异形体$J=1-0$线的积分时间很短,在很大的( )空间范围内已经可以获得很多关于总柱密度的信息。用于约束视觉消光的最佳线组不一定是信息量最大的单条线的组合。分子线更难实现对辐射场的精确约束。它们需要云表面发射的光谱线( 和 线)。这种方法可以更好地探索 ISM 代码提供的知识,并为未来的观测活动提供指导。
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来源期刊
Astronomy & Astrophysics
Astronomy & Astrophysics 地学天文-天文与天体物理
CiteScore
10.20
自引率
27.70%
发文量
2105
审稿时长
1-2 weeks
期刊介绍: Astronomy & Astrophysics is an international Journal that publishes papers on all aspects of astronomy and astrophysics (theoretical, observational, and instrumental) independently of the techniques used to obtain the results.
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