Molecular gas (mainly H2 molecule) and atomic gas (mainly HI atom) are very important baryonic components in interstellar medium, and they play significant roles in various kinds of physical processes in galaxies, including gas cooling and infall, star formation, metal producing, supernova reheating and feedback. It is generally considered that stars form in giant molecular clouds, and atomic gas is the reservoir of the molecular clouds.rnIn recent years, observations give more and more results on molecular and atomic gas with the development of observational technology.rnAtomic gas component in nearby galaxies at low redshift is observed through 21 cm radio emission by neutral hydrogen atoms. 21 cm HI surveys provide a lot of information about the neutral gas components in galaxies at low redshift. Some famous HI survey in recent years are HIPASS, HIJASS, WSRT, ALFALFA, THINGS etc. For galaxies at redshift higher than 0.2, people usually use DLA absorbers to observe the HI gas components indirectly.rnBecause of the symmetric structure, molecular hydrogen H2 components cannot be directly observed, and the molecular gas is observed through carbon monoxide or some other molecules as tracers. Some famous CO observations in recent years are FCRAO, COLD GASS, BIMA SONG, HERACLES. Based on these observations, people get the H2 properties for the local galaxies including the H2 mass functions at z = 0, the surface densityrnprofiles of molecular gas etc. Combining with the star formation rate observations of these galaxies, some astronomers find that the star formation rate correlates with the molecular components more tightly than total cold gas components.rnWith the advance of observational studies along this line, more and more galaxy formation models have include the molecular and neutral gas components. There are mainly two aspects on the transition of these two components. One is on the transition between atomic and molecular gas in ISM, and the other is the semi-analytic modeling of gas components in the galaxy evolution models.rnElmergreen et al, Blitz & Rosolowsky and some other work suggest that the molecular to neutral gas fraction is determined by the interstellar pressure. Krumholz, Mckee and Tumlinson made a model of the molecular gas formation and photo-ionization in interstellar gas, and they get the results that the molecular gas fraction can be approximately expressed as a function of the local gas surface density and gas metallicity.rnFor the semi-analytic modeling of gas components in galaxies, Obreschkow & Rawlings made the first attempts with the post-processing method based on the outputs of galaxy formation models, and Power et al. also did the similar work. But such kind of approach is not self-consistent, as their galaxy formation models do not contain the physical processes related with the neutral and molecular gas components. More recently, Cook et al, Lagos et al, and Fu et al. have included in their galaxy formation models with the transition of neutral and molecular gas and the molecular gas related star formation rate according to the findings in recent years. Their models are quite successful in predicting the observational resultsfor local galaxies such as the H2 and HI mass functions, the gas surface density profiles and some gas to star scaling relations. Their models are able to make additional observations.rnIn the future, observations by new facilities such as SKA, ALMA, ASKAP, MeerKAT can offer opportunities to get more information about the neutral and molecular gas, especially the direct detection of HI and molecular gas components in galaxies at high redshift, which can further help to study the cosmic evolution of cold gas and even galaxies.%星际介质中的分子气体(主要是分子氢H2)和原子气体(主要是中性氢HI)成分是星系中重子物质的重要组成部分,它们对于星系中的各种物理过程至关重要.近年来随着观测技术的提高,分子气体和原子气体的观测结果越来越多,其中中性氢主要依靠21cm氢原子射电辐射和DLA吸收体来观测,分子氢则通过一氧化碳作为示踪分子来探测.在这些观测结果基础上,理论工作者建立了一系列模型来解释和研究星际分子-原子气体的转换,分子气体与恒星形成,分子气体和原子气体在星系形成和演化中所起的作用等,并为将来更进一步的观测提供了指导和预言.
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