Implications of 21-cm observations for damped Ly α systems
Implications of 21-cm observations for damped Ly α systems
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DOI：
10.1046/j.1365-8711.2000.03793.x
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发表时间：
2000-10
影响因子：
4.8
通讯作者：
J. Chengalur;N. Kanekar
中科院分区：
文献类型：
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作者：
J. Chengalur;N. Kanekar
本文介绍了巨米波射电望远镜H_i 21厘米吸收观测结果，对候选和已证实的阻尼Lyα系统进行了观测。导出的自旋温度Tspin很高，在所有情况下都是1000 K或更高。我们还整理了从已发表的文献中的阻尼吸收器，其中21厘米的意见存在的列表，并讨论这些系统的性质的意见的影响。H i 21-cm剖面（跟踪冷气体）与低电离金属剖面（跟踪所有中性H i，冷和热）的交叉比较表明，在所有情况下，这两种光谱都是可用的，21-cm吸收速度与最深的金属线特征相一致。这与深金属线特征来自离散云的模型一致，但与最深特征是速度拥挤的结果的模型不一致。我们还发现阻尼Lyα系统的典型导出自旋温度明显高于银河系或附近的旋涡星系。唯一的例外是已知与螺旋盘有关的系统;事实上，这些系统确实显示出低的自旋温度。在多相介质中，导出的自旋温度是各个相的温度的加权平均值。因此，预计小的、低金属丰度的天体会有高的表观Tspin值，因为这些天体（根据银河系和高红移原星系中多相星际介质形成的现有理论模型）与大星系相比，在它们的多相星际介质中冷相的比例较低。因此，高T自旋与观测到的阻尼Lyα吸收体的低金属丰度相一致，也与最近的发现相一致，即阻尼吸收与各种星系类型有关（而不是局限于大螺旋的盘）。最后，虽然可观测到的系统数量很少，但我们认为可以确定以下趋势：在低红移时，阻尼Lyα吸收来自一系列系统，包括螺旋星系盘，而在高红移时，吸收主要发生在较小的系统中。
We present Giant Metrewave Radio Telescope H i 21-cm absorption observations, of candidate and confirmed damped Lyα systems. The derived spin temperatures Tspin are high, in all cases, ∼1000 K or higher. We have also collated from the published literature a list of damped absorbers for which 21-cm observations exist, and discuss the implications of the observations for the nature of these systems. A cross-comparison of the H i 21-cm profiles (which trace the cold gas) with the low ionization metal profiles (which trace all the neutral H i, both cold and warm) shows that in all cases for which both spectra are available, the 21-cm absorption coincides in velocity with the deepest metal line feature. This is consistent with models in which the deep metal line features arise from discrete clouds but not with models where the deepest features are the result of velocity crowding. We also find that the typical derived spin temperatures of damped Lyα systems are considerably higher than those in the Milky Way or nearby spiral galaxies. The only exceptions are systems that are known to be associated with the discs of spirals; these do, in fact, show low spin temperature. In a multi-phase medium the derived spin temperature is a weighted average of the temperatures of the individual phases. High apparent Tspin values are hence to be expected from small, low metallicity objects since these objects should (as per existing theoretical models of the formation of a multi-phase interstellar medium (ISM) in the Milky Way and high-redshift proto-galaxies) have a lower fraction of the cold phase in their ISM as compared with large galaxies. The high Tspin is hence consistent with the observed low metallicities of damped Lyα absorbers as well as with recent findings that damped absorption is associated with a variety of galaxy types (as opposed to being confined to the discs of large spirals). Finally, although the number of systems for which observations are available is small, we suggest that the following trend may be identified: at low redshift, damped Lyα absorption arises from a range of systems, including spiral galaxy discs, while, at high redshift, absorption occurs predominantly in smaller systems.