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arXiv 1607.04828astro-ph.GA
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利用21厘米吸收巡天测量遥远星系的平均HI自旋温度

Using 21-cm absorption surveys to measure the average HI spin temperature in distant galaxies

J. R. Allison, M. A. Zwaan, S. W. Duchesne, S. J. Curran

摘要:

我们提出了一个统计方法测量的平均HI自旋温度在遥远的星系使用预期的检测收益从未来的宽场21厘米吸收调查。作为一个示范性的案例研究,我们认为一个模拟的所有南方天空调查2小时,每指向与澳大利亚平方公里阵列探路者干预HI吸收在中间宇宙学红移之间的$z = 0.4$和$1$。例如,如果这样的调查产生$1000$吸收体,我们将推断在这些红移处阻尼莱曼$α$(DLAs)吸收体的谐波平均自旋温度为$\overline{T}_\mathrm{spin} \sim 100$ K,这表明这些系统中超过$50$ %的中性气体处于冷中性介质(CNM)中。相反,只有100个探测的较低的产额意味着$\overline{T}_\mathrm{spin} \sim 1000$ K和小于$10$ %的CNM分数。我们建议,这种方法可以用来提供独立的验证在最近的21厘米调查已知的DLAs在高红移的自旋温度演变和测量在中等红移低于$z \approx 1.7$的自旋温度。其中莱曼- $α$线路无法使用地面观测站进入。随着平方公里阵列探测的灵敏度越来越高,规模越来越大,对平均自旋温度的统计约束也越来越强。然而,这些最终将受到我们确定HI柱密度频率分布、覆盖因子和背景射电源群红移分布的精度的限制。

英文摘要:

We present a statistical method for measuring the average HI spin temperature in distant galaxies using the expected detection yields from future wide-field 21cm absorption surveys. As a demonstrative case study we consider a simulated all-southern-sky survey of 2-h per pointing with the Australian Square Kilometre Array Pathfinder for intervening HI absorbers at intermediate cosmological redshifts between $z = 0.4$ and $1$. For example, if such a survey yielded $1000$ absorbers we would infer a harmonic-mean spin temperature of $\overline{T}_\mathrm{spin} \sim 100$K for the population of damped Lyman $α$ (DLAs) absorbers at these redshifts, indicating that more than $50$ per cent of the neutral gas in these systems is in a cold neutral medium (CNM). Conversely, a lower yield of only 100 detections would imply $\overline{T}_\mathrm{spin} \sim 1000$K and a CNM fraction less than $10$ per cent. We propose that this method can be used to provide independent verification of the spin temperature evolution reported in recent 21cm surveys of known DLAs at high redshift and for measuring the spin temperature at intermediate redshifts below $z \approx 1.7$, where the Lyman-$α$ line is inaccessible using ground-based observatories. Increasingly more sensitive and larger surveys with the Square Kilometre Array should provide stronger statistical constraints on the average spin temperature. However, these will ultimately be limited by the accuracy to which we can determine the HI column density frequency distribution, the covering factor and the redshift distribution of the background radio source population.

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