天鹅座A的HI核周吸收盘
An HI absorbing circumnuclear disk in Cygnus A
摘要：
利用甚长基线阵列（VLBA）对强射电星系天鹅座A的核心区进行了HI吸收观测。这些数据显示了宽（FWHM = 231 \pm 21 km/s）和窄（FWHM <30 km/s）速度宽度吸收分量。宽速度吸收显示出对向射流的高不透明度，对核心的低不透明度和在射流侧没有吸收。我们认为，这些结果是最自然的解释由一个环核HI吸收盘取向大致垂直于射流轴。我们估计，HI吸收气体位于半径约80 pc处，刻度高度约为20 pc，密度n > 10^{4} cm^{-3}，总柱密度在10^{23}-10 ^{24} cm^{-2}范围内。HI吸收主要来自原子或分子气相的模型都可以拟合我们的数据。考虑到有效光束的建模表明，广泛的HI吸收气体成分并不覆盖天鹅座A的无线电核心，因此不会有助于阻止我们看到隐藏的类星体核的气柱。然而，如果从不同的方向观测天鹅座A，在~100 pc半径尺度上的盘气体将对核柱密度有显著的贡献，这意味着在某些射电星系中，这些尺度上的气体可能有助于中心引擎的遮蔽。我们认为，天鹅座A的环核环面包含的质量太少，无法为活动星系核提供超过10^{7}年的能量，但外部HI吸收气体盘中的物质可以提供一个储存库，为活动星系核提供燃料，并补充环面云。第二个窄HI吸收分量相对于系统速度有显著的红移（186 km/s），并且可能追踪最终将为源提供燃料的下落气体。[删节]
英文摘要：
We present Very Long Baseline Array (VLBA) HI absorption observations of the core region of the powerful radio galaxy Cygnus A. These data show both broad (FWHM = 231 \pm 21 km/s) and narrow (FWHM <30 km/s) velocity width absorption components. The broad velocity absorption shows high opacity on the counter jet, low opacity against the core and no absorption on the jet side. We argue that these results are most naturally explained by a circumnuclear HI absorbing disk orientated roughly perpendicular to the jet axis. We estimate that the HI absorbing gas lies at a radius of ~80 pc has a scale height of about 20 pc, density n > 10^{4} cm^{-3} and total column density in the range 10^{23}-10^{24} cm^{-2}. Models in which the HI absorption is primarily from an atomic or a molecular gas phase can both fit our data. Modelling taking into account the effective beam shows that the broad HI absorbing gas component does not cover the radio core in Cygnus A and therefore does not contribute to the gas column that blocks our view of the hidden quasar nucleus. If however Cygnus A were observed from a different direction, disk gas on ~100 pc radius scales would contribute significantly to the nuclear column density, implying that in some radio galaxies gas on these scales may contribute to the obscuration of the central engine. We argue that the circumnuclear torus in Cygnus A contains too little mass to power the AGN over > 10^{7} yr but that material in the outer HI absorbing gas disk can provide a reservoir to fuel the AGN and replenish torus clouds. The second narrow HI absorption component is significantly redshifted (by 186km/s) with respect to the systemic velocity and probably traces infalling gas which will ultimately fuel the source. [abridged]
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