Context. Level population inversion of hydrogen atoms in ionized gas may lead to stimulated emission of hydrogen recombination lines, and the level populations can in turn be affected by powerful stimulated emissions. Aims. In this work the interaction of the radiation fields and the level population inversion of hydrogen atoms is studied. The effect of the stimulated emissions on the line profiles is also investigated. Methods. Our previous nl-model for calculating level populations of hydrogen atoms and hydrogen recombination lines is improved. The effects of line and continuum radiation fields on the level populations are considered in the improved model. By using this method the properties of simulated hydrogen recombination lines and level populations are used in analyses. Results. The simulations show that hydrogen radio recombination lines are often emitted from the energy level with an inverted population. The widths of Hn$α$ lines can be significantly narrowed by strong stimulated emissions to be even less than 10 km s$^{-1}$. The amplification of hydrogen recombination lines is more affected by the line optical depth than by the total optical depth. The influence of stimulated emission on the estimates of electron temperature and density of ionized gas is evaluated. We find that comparing multiple line-to-continuum ratios is a reliable method for estimating the electron temperature, while the effectiveness of the estimation of electron density is determined by the relative significance of stimulated emission.
FAST search for circumstellar atomic hydrogen--I: the young planetary nebula IC 4997
利用中国贵州的500米口径球面射电望远镜（FAST）观测到年轻行星状星云IC 4997中21厘米的中性氢原子吸收。吸收来自也与Na I D线相关的壳层。H I壳的质量为$1.46\times10^{-2}$ M $_\odot$，动态年龄为990年。H I的柱密度估计为$7.1\times10^{20}$ cm $^{-2}$，这可以很好地解释为电离星云周围的光解离区域，受到H $_2$自屏蔽的限制。我们发现，原子电离氢的比例为0.6，这表明H I大大有助于整体星云质量。
Large-scale Map of Millimeter-wavelength Hydrogen Radio Recombination Lines around a Young Massive Star Cluster
我们报告了第一张大尺度（长度为10 pc）毫米波氢复合线（mm RRL）发射到W 43-Main年轻大质量星团（YMC）周围的巨大H II区域的地图。我们的毫米RRL数据来自IRAM 30米望远镜，并与来自Karl G. Jansky甚大阵列和HCO $^{+}$ 1-0线发射数据来自IRAM 30 m。毫米-RRLs揭示了一个膨胀的风吹电离气体壳层，其电子密度约为70-1500 cm $^{-3}$，由WR/OB团簇驱动，其产生的总Ly $α$光子通量为1.5 x 10 $^{50}$ s $^{-1}$。这个壳层与W 43-Main稠密云中的稠密中性分子气体相互作用。结合高光谱和角分辨率的mm-RRL和cm-RRL立方体，我们推导出电离气体发射的动力学和压力展宽的二维相对分布，并发现RRL线形主要由压力展宽（4-55 km s $^{-1}$）附近的YMC和动力学展宽（8-36 km s $^{-1}$）附近的壳的边缘。电离气体块托管超紧凑的H II区域发现在边缘的壳表明，大规模的电离气体运动触发了新的恒星生成的形成附近的外围的壳。
Constraints on the properties of warm ionized gas from low-frequency hydrogen radio recombination lines
Green Bank Observatory（格林班克天文台）；National Radio Astronomy Observatory（国家射电天文台）；Westminster College（威斯敏斯特学院）；Florida Space Institute, University of Central Florida（中佛罗里达大学佛罗里达航天研究所）；Arecibo Observatory（阿雷西博天文台）；Center for Advanced Research in Science and Engineering, University of Puerto Rico（波多黎各大学先进科学与工程研究中心）；West Virginia University（西弗吉尼亚大学）
Multiline observations of hydrogen, helium, and carbon radio-recombination lines toward Orion A: A detailed dynamical study and direct determination of physical conditions
JWST Measurements of Neutral Hydrogen Fractions and Ionized Bubble Sizes at $z=7-12$ Obtained with Ly$α$ Damping Wing Absorptions in 27 Bright Continuum Galaxies
Ly$α$ visibility from z = 4.5 to 11 in the UDS field: Evidence for a high neutral hydrogen fraction and small ionized bubbles at z $\sim$ 7
INAF – Osservatorio Astronomico di Roma（意大利国家天体物理研究所罗马天文台）；Dipartimento di Fisica, Università di Roma Sapienza（罗马大学物理系）；NSF’s National Optical-Infrared Astronomy Research Laboratory（美国国家光学-红外天文研究实验室）；Astrophysics Science Division, NASA Goddard Space Flight Center（NASA戈达德太空飞行中心天体物理学部）；Department of Astronomy, The University of Texas at Austin（德克萨斯大学奥斯汀分校天文学系）；Max Planck Institut für Astrophysik（马克斯·普朗克天体物理学研究所）；Institute for Astronomy, University of Edinburgh（爱丁堡大学天文学研究所）
Nuevos resultados sobre la cinemática global y nuclear de NGC 253: movimientos no circulares y emisión en Br-gamma
继续先前的研究（Cameroon et al. 2011），新的日心径向速度分布是从附近的星系NGC 253获得的电离氢复合线H-α。这些分布已经从长缝光谱学中推导出各种位置角。它还显示了日心径向速度分布对应的部分红外数据（电离氢复合线Br-γ）与凤凰摄谱仪的双子座南天文台观察。使用该仪器进行长缝连续映射将能够详细研究星系核心的运动学，该核心被尘埃强烈遮蔽。
The ionic composition of the local absorber towards 3C 273
通过光谱吸收特征，在我们星系的局部附近观察到热电离气体。最常见的假设是，这种气体在碰撞电离平衡中形成了围绕我们的银河系（MW）的光环。在本文中，我们调查的元素丰度的热和电离的本地气体。我们使用2.4 Ms的明亮blazar 3C 273的堆叠X射线光谱和探测的本地吸收功能。使用离子的X射线吸收线的离子拟合，我们推导出每个电离物种的柱密度。基于柱密度，我们重建了吸收测度分布（AMD），即作为温度的函数的类氢柱密度。我们报告了C、N、Ne和Fe相对于太阳O的元素丰度。先前对局部X射线发射谱线的测量以及目前的柱密度表明，热电离气体的标度高度为$1-80$ kpc，氢数密度为$10^{-4}-10^{-3}$ cm $^{-3}$。此外，我们还在类星体宽线区探测到类He O线，其速度为6400 $\pm$ 1500 km s $^{-1}$
A jet-gas interaction beyond the host galaxy: detection of a neutral hydrogen outflow at cosmic noon
Shanghai Astronomical Observatory, Chinese Academy of Sciences（中国科学院上海天文台）；School of Chemical and Physical Sciences, Victoria University of Wellington（惠灵顿维多利亚大学化学与物理科学学院）；First Light Fusion Ltd.（第一缕光融合有限公司）；Institute for Astronomy, University of Edinburgh（爱丁堡大学天体物理研究所）；Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town（开普敦大学天文学系数据密集型天文跨大学研究所）；International Centre for Radio Astronomy Research (ICRAR), Curtin University（科廷大学国际射电天文研究中心）；ATNF, CSIRO Space and Astronomy（澳大利亚联邦科学与工业研究组织空间与天文台射电天文国家设施）；State Key Laboratory of Radio Astronomy and Technology（射电天文与技术国家重点实验室）；College of Physics, Guizhou University（贵州大学物理学院）
