The accretion history of galactic haloes

我们提出了一个系统的研究中性和电离气体的物理性质在银河系的内部和外部晕。除了光晕中较大的中性气体络合物（分别为中速云和高速云；IVCs和hvc）之外，还有一群部分电离的气体结构，它们具有低气柱密度，具有很大的面积填充因子。这些结构的性质及其在银河系气体吸积史中的作用几乎是未知的。在本项目中，我们将把新的深HI 21cm数据与紫外光谱和光学光谱相结合，研究在HI柱密度低于1019 cm2的光晕中这种气体细丝的填充因子和尺寸分布。为此，我们希望建立一个包含400多条视线的银河系光晕吸收器的综合统计数据库。这将减少由于对感兴趣的云的距离模糊而自然引入的任何系统偏差。我们独特的数据样本将使我们能够真实地估计银河系的气体吸积率，并澄清目前未知的银河系晕云与低红移和高红移的类星体金属吸收线系统之间的关系。

We propose a systematic study of the physical properties of neutral and ionized gas in the inner and outer halo of the Milky Way. Beside the large neutral gas complexes in the halo (intermediateandhighvelocity clouds; IVCs and HVCs, respectively) there exists a population of partly ionized gaseous structures with low gas column densities that have a substantial area filling factor. The nature of these structures and their role in the Milky Way’s gas accretion history is nearly unexplored. In this project we will combine new, deep HI 21cm data with ultraviolet and optical spectra to study the filling factor and size distribution of such gaseous filaments in the halo at HI column densities below 1019 cm2. For this, we want to establish a comprehensive statistical data base of Milky Way halo absorbers, comprising more than 400 lines of sight. This will reduce any systematically bias naturally introduced by distance ambiguities towards the clouds of interest. Our unique data sample will allow us to realistically estimate the gas accretion rate of the Galaxy and to clarify the currently unknown relation between Milky Way halo clouds and intervening quasar metal absorptionline systems at low and high redshift.