HALOGAS星系及其环境的GBT调查I：揭示NGC891、NGC925、NGC4414和NGC4565周围HI的全貌
A GBT Survey of the HALOGAS Galaxies and Their Environments I: Revealing the full extent of HI around NGC891, NGC925, NGC4414 & NGC4565
摘要：
本文介绍了利用罗伯特C.伯德格林班克望远镜（GBT）由此产生的观测覆盖至少4度$^2$周围的这些星系的平均5 $σ$检测限为1.2 $\times$ 10 $^{18}$厘米$^{-2}$在20公里的速度范围内的$^{-1}$和9.1 $'$角标度。除了探测到与GBT数据相同的总通量外，GBT和原始Westerbork综合射电望远镜（WSRT）数据的空间分布在相同的空间分辨率下匹配良好。低于10 $^{19}$ cm $^{-2}$的HI柱密度的HI质量分数平均为2%。讨论了邻近旋涡星系低柱密度HI的可能起源。GBT没有新探测到相当数量的HI，这表明这些星系没有明显的扩散HI结构，并建议未来计划与SKA及其前体一起进行的调查必须深入到柱密度为10 $^{17}$ cm $^{-2}$的\textit{at least}，以显着增加探测与宇宙网和/或冷模吸积相关的HI的可能性。
英文摘要：
We present initial results from a deep neutral hydrogen (HI) survey of the HALOGAS galaxy sample, which includes the spiral galaxies NGC891, NGC925, NGC4414, and NGC4565, performed with the Robert C. Byrd Green Bank Telescope (GBT). The resulting observations cover at least four deg$^2$ around these galaxies with an average 5$σ$ detection limit of 1.2$\times$10$^{18}$ cm$^{-2}$ over a velocity range of 20 km s$^{-1}$ and angular scale of 9.1$'$. In addition to detecting the same total flux as the GBT data, the spatial distribution of the GBT and original Westerbork Synthesis Radio Telescope (WSRT) data match well at equal spatial resolutions. The HI mass fraction below HI column densities of 10$^{19}$ cm$^{-2}$ is, on average, 2\%. We discuss the possible origins of low column density HI of nearby spiral galaxies. The absence of a considerable amount of newly detected HI by the GBT indicates these galaxies do not have significant extended diffuse HI structures, and suggests future surveys planned with the SKA and its precursors must go \textit{at least} as deep as 10$^{17}$ cm$^{-2}$ in column density to significantly increase the probability of detecting HI associated with the cosmic web and/or cold mode accretion.
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