星际介质中的气体星云被炽热天体发出的高能紫外辐射电离、加热，形成光致电离气体星云（行星状星云、电离氢区、星暴星系等）。光致电离气体星云的观测和分析是获取银河系、特别是河外星系中重元素含量和分布的重要手段，为研究恒星及大爆炸核合成过程、星系恒星形成和化学增丰历史提供了的关键性数据。本项目组在光致电离气体星云的深度分光观测、理论分析和数值模拟方面发表了一系列重要工作，系统发展了基于重元素离子光学复合线的等离子体诊断和丰度分析手段，在国际上有重要的影响。

Ionized and heated by strong ultraviolet radiation fields, emission line nebulae (ELNe; including planetary nebulae, H II regions, starburst galaxies, etc.) glow with strong emission lines. Observations and analyses of ELNe allow us to probe the contents and distributions of heavy elements within and beyond the Milky Way galaxy, and to obtain vital information of the stellar and Big Bang nucleosynthesis, and of the star formation and chemical enrichment history of galaxies. Being a world-leading group in the field, we have done extensive investigations on planetary nebulae and H II regions, including deep spectroscopic observations, theoretical analysis and numerical simulations. Particularly, we have developed systematic tools of plasma diagnostics and abundance determinations based on heavy element optical recombination lines. Our series influential studies have driven the development of this active field.

星际介质在恒星的形成、星系的化学演化方面起着重要的作用；星际尘埃对星光的吸收、散射和再辐射改变了天体的颜色和出射能谱，从而影响我们对银河系和星系结构的认识, 因此对星际消光的改正是还原天体颜色和亮度的基础；星际弥散带(DIB)源自星际介质，自1921年发现至今为止，人们还没有确定任何一条DIB的载体。本团组基于国家大科学装置LAMOST望远镜获取的世界上最大、性质均一的光谱数据库，结合国内外其他测光与分光数据，在银河系星际尘埃三维分布、银河系消光规律、银河系气体与尘埃关系方面发表了一些列重要的工作，受到国内外学者的广泛引用，在国际上有较高的知名度。

Interstellar medium (ISM) plays an important role in star formation and chemical evolution of galaxies. Interstellar dust absorbs and scatters ultraviolet to infrared (IR) light, and re-emits the absorbed energy thermally in the mid-IR to far-IR (FIR) bands. These processes reshape the radiation field of galaxies. Understanding their effects is therefore crucially important in revealing the physical properties of astronomical objects. The diffuse interstellar bands (DIBs) show up as absorption troughs in the optical/near infrared (IR) wavelength range against the continuous spectra of hot background stars . Since their first detection in 1921, the origin of DIBs is still not fully understood. Based on the world largest stellar spectra library from LAMOST Regular Galactic Surveys, we have studied the optical and IR extinction law, the Galactic 3D dust extinction distribution, as well as the dust and gas correlations.

恒星形成是宇宙中的一个基本物理过程，是联系星际介质物理，银河系结构，星系形成和演化，行星形成等重要领域的基础。我们对星际介质中恒星形成的研究包括气体星云的多尺度塌缩，恒星形成和银河系结构的关系以及不同星系中星际介质的物理状态。通过对银河系中大尺度纤维结构的认证，我们证实了银河系旋转对秒差距尺度分子云动力学的强烈影响。结合观测数据、理论分析以及数值计算，我们对银河系中星际介质的动力学和恒星形成过程开展深入的研究，进而研究星系的宇宙学演化。

Star formation is one of the fundamental astrophysical processes shaping the observable universe. It plays the key role in studies of ISM physics, Milky Way structure, evolution of galaxies and the formation of planets. In this area of research, we focus on multi-scale collapse of molecular cloud, connecting Milky Way structure to star formation, and ISM condition in different galaxies. We have identified and studied large-scale filamentary structures in the Milky Way disk, and shown that the dynamics of pc-scale molecular cloud is strongly influenced by the Galactic shear. By combining observational data with theoretical and numerical analyses, we are carrying out studies for the star formation process in the Milky Way, aiming to connect our findings to the cosmic evolution of disk galaxies.

代表性论文(Selected publications):

• Liu, X.-W.; Storey, P. J.; Barlow, M. J.; Danziger, I. J.; Cohen, M.; Bryce, M., "NGC 6153: a super-metal-rich planetary nebula?", 2000, MNRAS, 312, 585.

• Liu, X.-W., "The dark secrets of gaseous nebulae -- highlights from deep spectroscopy", 2010, New Vision 400: Engaging Big Questions in Astronomy and Cosmology Four Hundred Years after the Invention of the Telescope, arXiv:1001.3715.

• Chen, B.-Q.; Liu, X.-W.; Ren, J.-J.; Yuan, H.-B.; Huang, Y.; Yu, B.; Xiang, M.-S.; Wang, C.; Tian, Z.-J.; Zhang, H.-W., "Mapping the three-dimensional dust extinction towards the supernova remnant S147 - the S147 dust cloud", 2017, MNRAS, 472, 3924.    

• Chen, B.-Q.; Liu, X.-W.; Yuan, H.-B.; Zhang, H.-H.; Schultheis, M.; Jiang, B.-W.; Huang, Y.; Xiang, M.-S.; Zhao, H.-B.; Yao, J.-S.; Lu, H., "A three-dimensional extinction map of the Galactic anticentre from multiband photometry", 2014, MNRAS, 443, 1192.

• Liu, X.-W.; Barlow, M. J.; Cohen, M.; et al., "ISO LWS observations of planetary nebula fine-structure lines", 2001, MNRAS, 323, 343. 

• Yuan, H. B.; Liu, X. W.; Xiang, M. S., "Empirical extinction coefficients for the GALEX, SDSS, 2MASS and WISE passbands", 2013, MNRAS, 430, 2188.

• Li, G.-X.; Wyrowski, F.; Menten, K.; Belloche, A., "A 500 pc filamentary gas wisp in the disk of the Milky Way", 2013, A&A, 559, A34.

• Li, G.-X.; Urquhart, J. S.; Leurini, S.; Csengeri, T.; Wyrowski, F.; Menten, K. M.; Schuller, F., "ATLASGAL: A Galaxy-wide sample of dense filamentary structures", 2016, A&A, 591, A5.

• Li, G.-X.; Burkert, A., "Constructing multiscale gravitational energy spectra from molecular cloud surface density PDF - interplay between turbulence and gravity", 2016, MNRAS, 461, 3027.

• Li, G.-X., "Criteria for gravitational instability and quasi-isolated gravitational collapse in turbulent medium", 2017, MNRAS, 465, 667.