低红移河外星系中性氢原子气体观测

Neutral atomic hydrogen observation of low redshift galaxies

我们回顾了近年来近邻宇宙星系中HI的研究进展,包括星系怎样吸积HI气体、HI在星系中的含量和分布、HI跟恒星形成率的关系以及环境对星系中HI的影响.近邻宇宙中仍然没有发现直接的证据支持HI吸积,说明这个过程很缓和.星系团、组尺度的环境以及局部星系密度环境对星系中HI含量有很明显的影响,然而我们仍不清楚实现这一过程的具体细节.不同星系内HI的分布具有很多共性,例如HI质量-直径关系,HI盘在光学盘外沿弯曲的普遍性,然而从宇宙学下的星系形成模型上理解这些共性仍然是个难题.在没有可探测的分子气体的低气体面密度区,HI与恒星形成活动有密切联系,表现在恒星形成覆盖面积比例和恒星形成面密度都与HI面密度相关.然而HI和恒星形成之间的联系怎样被激发以及被哪些物理条件约束仍然未明.相信接下来几年到几十年里,射电数据量的大爆炸会让一些未解之谜逐渐明朗.

We review recent studies on the atomic hydrogen gas（HI） in galaxies at low redshift. The topics include how HI is accreted onto galaxies, how it is distributed in galaxies and related to the other galaxy properties, how it is used to form stars, and how it is affected by environmental effects. There is no direct evidence for on-going gas accretion at low redshift, implying that gas accretion has become gentle and hard to trace compared to at the high redshift. There is strong evidence that both the cluster/group environments and local galaxy number densities affect the HI content and further affect the star formation in galaxies, but how and when these happen still remain unclear to us. The distributions of HI in different galaxies have many common features, like the HI size-mass relation and the prevalence of HI warps. However we do not fully understand the physical drivers for these common features. At low gas column densities in galaxies, where the molecular gas is hardly detectable, HI seems to be a major regulator on star forming efficiency. Both the star forming region covering fraction and the star formation rate surface density correlate with the HI surface density. However it remains unclear how HI is turned to stars and regulated by other physical properties.We believe that the coming large HI surveys will shed light on these questions.