标题： 活动星系核中的巨脉泽盘 显示英文标题

标题： Megamaser Disks in Active Galactic Nuclei

摘要： 最近对明亮的河外水脉泽源的光谱和VLBI成像观测显示，巨脉泽辐射通常起源于活动星系核（AGN）中心附近的薄环核盘。 利用一般的辐射和运动学考虑，并考虑到观测到的流量变化，我们认为脉泽辐射区域是成团的，这一结论与驱动碰撞泵浦脉泽的详细机制（如X射线加热、冲击等）无关。 我们研究了这样的情景，其中团块代表离散的气体凝聚体（即云），而不仅仅对应于盘中的速度不规则性。 我们表明，即使两个云在视线方向上的速度相干长度内重叠，也可以通过自放大作用解释像NGC 4258和NGC 1068这样的源中高速度“卫星”特征的全部脉泽流量，并且我们认为云的自放大很可能也对这些天体中背景放大的“系统”特征的流量有所贡献。 类似的解释之前已被提出用于银河系形成恒星区域的水脉泽源。 我们认为这个图景为河外巨脉泽源的时间变化特性及其与塞弗特2型星系的明显关联提供了一个自然的解释。 我们还表明，所需的云空间密度和内部密度与核（宽发射线区类型）云的典型值是一致的。 显示英文摘要

摘要： Recent spectroscopic and VLBI-imaging observations of bright extragalactic water maser sources have revealed that the megamaser emission often originates in thin circumnuclear disks near the centers of active galactic nuclei (AGNs). Using general radiative and kinematic considerations and taking account of the observed flux variability, we argue that the maser emission regions are clumpy, a conclusion that is independent of the detailed mechanism (X-ray heating, shocks, etc.) driving the collisionally pumped masers. We examine scenarios in which the clumps represent discrete gas condensations (i.e., clouds) and do not merely correspond to velocity irregularities in the disk. We show that even two clouds that overlap within the velocity coherence length along the line of sight could account (through self-amplification) for the entire maser flux of a high-velocity ``satellite'' feature in sources like NGC 4258 and NGC 1068, and we suggest that cloud self-amplification likely contributes also to the flux of the background-amplifying ``systemic'' features in these objects. Analogous interpretations have previously been proposed for water maser sources in Galactic star-forming regions. We argue that this picture provides a natural explanation of the time-variability characteristics of extragalactic megamaser sources and of their apparent association with Seyfert 2-like galaxies. We also show that the requisite cloud space densities and internal densities are consistent with the typical values of nuclear (broad emission-line region-type) clouds.