Hydrogen is the most abundant element in the universe and contributes to about 76% of the baryonic matter. Hydrogen in neutral state (conventionally called neutral hydrogen, HI) is widespread in the universe. It is the building material of stars and galaxies.The study of neutral hydrogen has a long history. With the improvement of sensitivity and angular resolution of radio telescopes, great advances have been made in this area. Some hot topics in neutral hydrogen studies are sketched in this paper, including the influence of neutral hydrogen on soft X-ray observations, neutral hydrogen distance measurement, neutral hydrogen content in molecular clouds, and studies of dark matter and large scale structure of the universe using the HI 21 cm line emission. This paper can serve as a short introduction to neutral hydrogen studies for those researchers who are not familiar with this area.Neutral hydrogen also plays an important role in molecular clouds, the birth places of stars. Recently, it is found that HI narrow self-absorption (HINSA) can be used to estimate the HI abundance in molecular clouds, which can further be used to constrain the age of the molecular cloud. Another related hot topic is how H2 forms out of HI gas. Great efforts have been made to search for molecular clouds in transition state, in which the conversion from HI to H2 is still on-going. The H2-HI phase transition is a key step towards understanding star formation in the early stage.The distribution of neutral hydrogen is well extended in galaxies. Neutral hydrogen is a good tracer for studying the dynamics of galaxies and the distribution of dark matter. The global mass distribution and physical states of the neutral hydrogen have been well established in the Milky Way and some nearby galaxies. Investigations are now focused on the HI distribution in the halo or environment of the Milky Way and the some nearby galaxies. These studies can reveal the dynamical properties of the host galaxies, and potentially help to distinguish between different dark matter models. High resolution HI observations are also suitable for the studies of the interaction and merging of galaxies.Compared with the stellar component, neutral hydrogen in galaxies is a better tracer of the large scale structure of the universe. It is not affected by extinction and star formation history and hence observations of the HI emission is expected to yield a less biased picture of the large scale structure of the universe. A new technique, namely intensity mapping, for observing HI at moderate redshift is being tested at the GBT. Such observations can potentially extended the neutral hydrogen studies to a much higher redshift.%氢元素是宇宙中重子物质的一种主要成分.处于中性状态的原子氢(通常简称为中性氢)在宇宙中分布广泛,是恒星和星系的物质基础.中性氢是分子云中的一种重要成分.它在星系中的分布广泛而延展,是研究星系的物质分布、动力学以及暗物质性质很好的示踪物.中性氢的研究已经有较长的历史,随着射电望远镜灵敏度和角分辨率的提高,正在形成越来越多对中性氢的新认识.对中性氢的进一步研究对其他一些天体物理研究也有重要意义.概括中性氢研究的几个前沿问题,包括研究中性氢对软X射线观测的影响,用银河系中性氢的动力学模型限定天体距离,研究分子云中的中性氢成分,用中性氢研究星系的动力学和星系中的暗物质分布,用高分辨率的中性氢观测研究星系的相互作用以及用中性氢研究宇宙的大尺度结构.对不熟悉中性氢研究的研究者而言,这可以作为了解这个领域的一个参考.
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