基于FAST对连续谱点源的吸收线进行银河系暗气体研究

星际气体演化是天文领域重大科学问题。近年来的多波段观测表明存在多余的未被中性氢及一氧化碳辐射所示踪的暗气体。暗气体的存在挑战着我们对于星际介质气体从原子到分子演化过程的理解。对连续谱点源的吸收线是测量暗气体精确物理状态的绝佳观测方法，但已有的观测主要集中在反银心及高银纬区域。与这些区域相比，银河系第一象限银道面气体具有完全不同的外部环境，如紫外辐射场强度。对星际气体的观测研究是五百米口径球面射电望远镜(FAST)的核心科学目标之一。本项目计划进行两方面的工作：1，分析FAST已经规划的"FAST多科学目标同时扫描巡天"项目中漂移扫描强连续谱点源中性氢数据,研究银河系中光厚中性氢的分布及其对暗气体的贡献；2，申请利用FAST、Arecibo及VLA望远镜对银河系第一象限银道面附近50个连续谱点源进行中性氢及羟基吸收线观测，研究暗气体在银河系内的物理特性、化学演化及其受周围物理环境的影响。

The evolution of interstellar gas is a significant scientific problem in astronomy. Multi-band observations in recent years revealed the existence of dark gas, which is not traced by atomic hydrogen (HI) and carbon monoxide (CO). The presence of dark gas make it a challenge of our understanding of atomic-molecular transition in interstellar medium (ISM). Absorption observation toward continuum source is the most effective method to measure accurate physical properties of Galactic dark gas. The current absorption observations focus on anti-galactic and high Galactic latitude regions. Compared to ISM in these regions, the ISM in the Galactic plane of first quadrant of the Milky Way is undergoing different external environments (e.g., intensity of UV radiation). Investigating interstellar gas is one of main science goals of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This project aims to do two kinds of studies: 1, Analyzing the HI drifting scan data across strong continuum sources in CRAFTS (The Commensal Radio Astronomy FAST Survey) project, which is already scheduled by FAST. The analysis will focus on spatial distribution of optical thick HI in the Milky Way and the contribution of optically thick HI to dark gas. 2, Applying FAST, Arecibo and VLA observations of HI and OH absorption toward 50 continuum sources around the Galactic plane of first quadrant of the Milky to investigate accurate physical properties, chemical evolution and environment effect of Galactic dark gas.