标题： 银河系晕中低柱密度气体团块的高分辨率21厘米观测 显示英文标题

标题： High-resolution 21-cm observations of low-column density gas clumps in the Milky Way halo

摘要： 我们基于高分辨率21厘米观测研究了银河系晕中的低列密度气体团块的性质。 利用WSRT和VLA的干涉数据，我们研究了四条指向类星体视线上的低、中、高径向速度处的氢原子（HI）发射。 在这几条视线中，我们之前在光学光谱中检测到了弱的CaII和NaI吸收体。 高分辨率HI数据的分析揭示了几个速度与光学吸收相似的紧凑且寒冷的中性气体团块。这些团块的HI线宽在1.8到13公里/秒之间，对应的气体动能温度上限为70到3700开尔文。中性气体的HI列密度范围在5E18到3E19 1/平方厘米之间。 所有团块的角直径仅为几分钟弧度。 我们的高分辨率21厘米观测表明，我们在光学类星体光谱中看到的许多CaII和NaI吸收体与小角尺度上的低列密度HI团块相关联。 这表明除了晕中的大质量、高列密度的中性气体云（常见的21厘米局部晕气体LVCs、中间晕气体IVCs和高晕气体HVCs）外，还存在一个低质量、中性气体结构的群体，在现有的IVCs和HVCs的全天空21厘米巡天中大多未被发现。 所探测到的HI团块的热气压与银河系内晕和外晕气体的理论模型预测一致。 显示英文摘要

摘要： We study the properties of low-column density gas clumps in the halo of the Milky Way based on high-resolution 21-cm observations. Using interferometric data from the WSRT and the VLA we study HI emission at low-, intermediate- and high radial velocities along four lines of sight towards quasars. Along these sightlines we previously detected weak CaII and NaI absorbers in their optical spectra. The analysis of the high-resolution HI data reveals the presence of several compact and cold clumps of neutral gas at velocities similar to the optical absorption. The clumps have narrow HI line widths in the range of 1.8 to 13 km/s, yielding upper limits for the kinetic temperature of the gas of 70 to 3700 K. The neutral gas has low HI column densities in the range of 5E18 to 3E19 1/cm^2. All clumps have angular sizes of only a few arcminutes. Our high-resolution 21-cm observations indicate that many of the CaII and NaI absorbers seen in our optical quasar spectra are associated with low-column density HI clumps at small angular scales. This suggests that next to the massive, high-column density neutral gas clouds in the halo (the common 21-cm LVCs, IVCs, and HVCs) there exists a population of low-mass, neutral gas structures in the halo that remain mostly unseen in the existing 21-cm all-sky surveys of IVCs and HVCs. The estimated thermal gas pressures of the detected HI clumps are consistent with what is expected from theoretical models of gas in the inner and outer Milky Way halo.