AI 中文总结
该研究利用FLASH数据及ASKAP的IPS测量，在特定区域对157个射电源进行HI吸收搜索，通过NSI判断源的致密性，检测到两条新HI吸收线，表明ASKAP的IPS测量能为识别致密射电源样本提供有力工具。
AI 中文摘要
在这项初步研究中，我们使用来自首次大型HI吸收调查（FLASH）的数据，在澳大利亚平方公里阵列探路者（ASKAP）射电望远镜观测的单个区域内，对157个明亮射电源进行红移HI 21cm吸收搜索，这些源在820MHz有行星际闪烁（IPS）测量，红移范围为0.4<z<1。ASKAP的IPS测量总共仅使用2.5分钟观测时间，能让我们在712 - 1000MHz的FLASH频段内，在亚角秒尺度上估计这些射电源的致密性。每个源的归一化闪烁指数（NSI）反映了直径小于0.1角秒的致密成分产生的通量密度比例。我们发现，通量密度高于150mJy的ASKAP源中，18±5%是高度致密的，NSI≥0.8，这意味着它们至少80%的射电辐射来自大小小于约800pc的单个区域。这些源的致密性使它们成为HI吸收搜索的理想探针。我们还利用FLASH在IPS数据覆盖的约30平方度天空区域中，检测到两条针对致密源的新HI吸收线，一条在MRC 2125 - 237（NSI = 0.98）中红移z = 0.9540的相关HI线（与光学红移匹配），另一条在MRC 2131 - 241（NSI = 0.84）中可能的z = 0.4632的居间线。这些初步结果表明，ASKAP的IPS测量可为在几百MHz频率下识别大面积天空中的致密射电源均匀样本提供一个简单而强大工具。
英文摘要
In this pilot study, we use data from the First Large Absorption Survey in HI (FLASH) to search for redshifted HI 21cm absorption at $0.4<z<1$ towards 157 bright radio sources with Interplanetary Scintillation (IPS) measurements at 820MHz in a single field observed with the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope. The ASKAP IPS measurements, which use only 2.5 minutes of observing time in total, allow us to estimate the compactness of these radio sources on sub-arcsecond scales at a frequency within the 712-1000MHz FLASH band. In particular, the Normalised Scintillation Index (NSI) for each source reflects the fraction of the flux density arising from compact components less than 0.1 arcsec in diameter. We find that $18\pm5$% of ASKAP sources with flux densities above 150mJy are highly compact with NSI $\geq0.8$ - implying that at least 80% of their radio emission arises from a single region smaller than about 800pc in size. The compactness of these sources makes them ideal probes for an HI absorption search, since the covering factor for any HI gas clouds along the line of sight is likely to be high. About half of the sources with NSI $\geq0.8$ also have peaked radio spectral energy distributions (SEDs), consistent with previous IPS studies at lower frequencies. These pilot results imply that IPS measurements with ASKAP can provide a simple and powerful tool for identifying uniform samples of compact radio sources at frequencies of a few hundred MHz across large areas of sky. With FLASH, we detect two new HI absorption lines against compact sources in the $\sim30$deg$^2$ region of sky covered by the IPS data; an associated HI line at redshift $z=0.9540$ (with a matching optical redshift) in MRC 2125-237 (NSI = 0.98), and a likely intervening line at $z=0.4632$ towards MRC 2131-241 (NSI = 0.84).
Comments26 pages, 11 figures, 10 tables, accepted for publication in PASA
