Milky Way晕面外气体的CaII和NaI吸收特征
CaII and NaI absorption signatures from extraplanar gas in the halo of the Milky Way
摘要：
分析了面外中高速气体的吸收特性和物理条件，研究了中性弱电离的银河系晕气体的分布及其与银河系和其他旋涡星系演化的关系。我们结合光学吸收线测量CaII/NaI和21厘米发射线观测HI沿103河外视线对类星体（类星体）和活动星系核（AGN）。档案光学光谱是在欧洲南方天文台甚大望远镜上用紫外线辐射获得的，而21厘米高的观测是在埃费尔斯堡用100米射电望远镜进行的。UVES光谱的分析表明，在中等和高速度的单组分和多组分CaII/NaI吸收剂存在于约35%的视线中，表明存在中性面外气体结构。在某些情况下，CaII/NaI吸收与HI中速或高速气体有关，而其他吸收剂则没有相关的HI排放。我们的研究表明，银河系晕中充满了大量的中性气体结构，其高柱密度尾代表了在21厘米的调查中看到的常见HI高速云的人口。CaII柱密度分布遵循幂律，这是相媲美的中间金属线系统对类星体的分布。许多的统计和物理性质的钙吸收类似的强镁吸收系统中观察到的其他星系的环星系环境，这表明这两个吸收人口可能是密切相关的。
英文摘要：
We analyse absorption characteristics and physical conditions of extraplanar intermediate- and high-velocity gas to study the distribution of the neutral and weakly ionised Milky Way halo gas and its relevance for the evolution of the Milky Way and other spiral galaxies. We combine optical absorption line measurements of CaII/NaI and 21 cm emission line observations of HI along 103 extragalactic lines of sight towards quasars (QSOs) and active galactic nuclei (AGN). The archival optical spectra were obtained with UVES at the ESO VLT, while the 21 cm HI observations were carried out using the 100-m radio telescope at Effelsberg. The analysis of the UVES spectra shows that single and multi-component CaII/NaI absorbers at intermediate and high velocities are present in about 35% of the sight lines, indicating the presence of neutral extraplanar gas structures. In some cases the CaII/NaI absorption is connected with HI intermediate- or high-velocity gas, while other absorbers show no associated HI emission. Our study suggests that the Milky Way halo is filled with a large number of neutral gaseous structures whose high column density tail represents the population of common HI high-velocity clouds seen in 21 cm surveys. The CaII column density distribution follows a power-law which is comparable to the distribution found for intervening metal-line systems toward QSOs. Many of the statistical and physical properties of the CaII absorbers resemble those of strong MgII absorbing systems observed in the circumgalactic environment of other galaxies, suggesting that both absorber populations may be closely related.
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