AI 中文总结
本研究通过GBT与WSRT观测对比，发现NGC891和NGC4565的内部中性CGM无方位角对称性，离轴HI含量远高于主/次轴，证实了真实CGM探测并揭示其与HI盘滞后共转。
AI 中文摘要
此前研究表明，NGC891和NGC4565星系的星系晕（CGM）中的弥散HI发射，沿次轴指向追踪流入运动，沿主轴指向则与HI盘共转，延伸至约100千秒差距（Das2020b,Das2024a）。为获得内部中性CGM的360°视图（NGC891小于25千秒差距，NGC4565小于30千秒差距），我们利用绿岸望远镜（GBT）沿主轴间45°的离轴方向开展深度凝视观测，在20千米每秒的速度宽度上达到1.1-1.2×10^17厘米^-2的5σ柱密度灵敏度。尽管用单碟望远镜探测内部CGM的HI发射较为常见，但将真实CGM发射与盘污染分离极为困难，且此前基本未成功。为实现这一目标，我们将单碟探测结果与荷兰韦斯特博克综合射电望远镜（WSRT）HALOGAS巡天的深度干涉图进行对比，并通过纳入速度偏移校正和通道-wise亮温度缩放改进了此前的方法。我们发现，在NGC891和NGC4565中，GBT探测到的发射分别有30%-38%和18%-28%无法用WSRT解释，这意味着探测到了真实的CGM信号。离轴方向的HI含量是主（次）轴方向的4-6（3-7）倍，否定了中性CGM具有方位角对称性的普遍假设。两个星系的弥散内部CGM的速度轮廓表明其与HI盘存在滞后共转。本研究展示了深度观测与跨仪器仔细对比在表征CGM中弥散HI方面的效力。
英文摘要
Diffuse HI emission in the circumgalactic medium (CGM) of NGC891 and NGC4565 has been previously shown to trace an inflow along minor axes pointings and to co-rotate with the HI disk along major axes pointings out to ~100 kpc (Das2020b,Das2024a). To obtain a 360$^\circ$ view of the inner neutral CGM ($\rm < 25 kpc$ for NGC891, $\rm < 30 kpc$ for NGC4565), we perform deep stare observations with the Green Bank Telescope (GBT) along the off-axes, 45$^\circ$ between principal axes, achieving a 5$σ$ column density sensitivity of $1.1-1.2 x 10^{17} \rm cm^{-2}$ over a 20 kms$^{-1}$ velocity width. While detecting HI emission in the inner CGM with single-dish telescopes is common, separating the true CGM emission from disk contamination is extremely challenging and has so far been largely unsuccessful. To achieve that, we compare our single-dish detections to deep interferometric maps from the Westerbork Synthesis Radio Telescope (WSRT) HALOGAS survey, and improve upon our previous methods by incorporating velocity offset corrections and channel-wise brightness-temperature scaling. We find that $30-38$ % and $18-28$ % of the emission detected by the GBT cannot be explained by WSRT in NGC891 and NGC4565, respectively, implying a true CGM detection. There is $4-6$ ($3-7$) times more HI along the off-axes than major (minor) axes, nullifying the common assumption of azimuthal symmetry of the neutral CGM. The velocity profile of the diffuse inner CGM suggests a lagged co-rotation with the HI disk in both galaxies. This exercise illustrates the power of deep observation and careful cross-instrument comparisons to characterize the diffuse HI in the CGM.
CommentsAccepted to ApJ. 18 pages and 9 figures
