ALFALFA中性氢速度宽度函数的南北不对称性
The north-south asymmetry of the ALFALFA HI velocity width function
AI总结：
针对ALFALFA 21厘米巡天发现的中性氢宽度函数南北数密度差异与Λ冷暗物质模型预测的矛盾，本研究通过模拟巡天揭示差异源于巡天灵敏度差异和1/V_eff算法未完全校正空间成团偏差，并给出校正后的更新测量结果，为用HIWF检验宇宙学模型扫清系统误差障碍。
摘要：
河外21厘米射电源的数密度作为其谱线宽度的函数——即中性氢宽度函数（HIWF）——是暗物质晕质量函数（HMF）的灵敏示踪剂。Λ冷暗物质模型预测，只要在足够大的体积中采样，HMF在各处都应是相同的，这意味着HIWF也应如此。ALFALFA 21厘米巡天在银河系北天和南天天区测量了HIWF，发现北天的数密度系统性更高。从表面上看，这与理论预测存在矛盾。我们使用Sibelius-DARK N体模拟和半解析星系形成模型GALFORM构建了一个模拟ALFALFA巡天。我们发现数密度的偏移有两个来源：两个天区的巡天灵敏度不同，这一点在之前的测量中未被正确考虑；此外，用于完备性校正的1/V_eff算法未能完全解释星系分布中空间成团性带来的偏差。后者主要由北天天区30 Mpc范围内的前景超密度驱动，但更遥远的结构也起到了一定作用。我们提供了经巡天灵敏度变化校正后的ALFALFA HIWF（以及中性氢质量函数HIMF）的更新测量结果。只有当这类系统效应被理解并校正后，才能利用HIWF对宇宙学模型进行检验。
英文摘要：
The number density of extragalactic 21-cm radio sources as a function of their spectral line-widths -- the HI width function (HIWF) -- is a sensitive tracer of the dark matter halo mass function (HMF). The $Λ$ cold dark matter model predicts that the HMF should be identical everywhere provided it is sampled in sufficiently large volumes, implying that the same should be true of the HIWF. The ALFALFA 21-cm survey measured the HIWF in northern and southern Galactic fields and found a systematically higher number density in the north. At face value, this is in tension with theoretical predictions. We use the Sibelius-DARK N-body simulation and the semi-analytical galaxy formation model GALFORM to create a mock ALFALFA survey. We find that the offset in number density has two origins: the sensitivity of the survey is different in the two fields, which has not been correctly accounted for in previous measurements; and the $1/V_{\mathrm{eff}}$ algorithm used for completeness corrections does not fully account for biases arising from spatial clustering in the galaxy distribution. The latter is primarily driven by a foreground overdensity in the northern field within $30\,\mathrm{Mpc}$, but more distant structure also plays a role. We provide updated measurements of the ALFALFA HIWF (and HIMF) correcting for the variations in survey sensitivity. Only when systematic effects such as these are understood and corrected for can cosmological models be tested against the HIWF.
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