Megamaser disks provide the most precise and accurate extragalactic supermassive black hole (BH) masses. Here we describe a search for megamasers in nearby galaxies using the Green Bank Telescope (GBT). We focus on galaxies where we believe that we can resolve the gravitational sphere of influence of the BH and derive a stellar or gas dynamical measurement with optical or NIR observations. Since there are only a handful of super massive BHs that have direct BH mass measurements from more than one method, even a single galaxy with a megamaser disk and a stellar dynamical BH mass would provide necessary checks on the stellar dynamical methods. We targeted 87 objects from the Hobby–Eberly Telescope Massive Galaxy Survey, and detected no new maser disks. Most of the targeted objects are elliptical galaxies with typical stellar velocity dispersions of 250 km s?1 and distances within 130 Mpc. We discuss the implications of our non-detections, whether they imply a threshold X-ray luminosity required for masing, or possibly reflect the difficulty of maintaining a masing disk around much more massive ( ) BHs at a low Eddington ratio. Given the power of maser disks for probing BH accretion and demographics, we suggest that future maser searches should endeavour to remove remaining sample biases, in order to sort out the importance of these covariant effects.
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机译：Megamaser盘提供了最精确和最精确的银河系超质量黑洞（BH）质量。在这里，我们描述了使用Green Bank望远镜（GBT）搜索附近星系中的大质量物质。我们专注于我们认为可以解决BH的重力影响范围并通过光学或NIR观测得出恒星或气体动力学测量结果的星系。由于只有极少数的超大型BH可以通过多种方法直接测量BH的质量，因此，即使是只有一个具有巨型脉冲盘和恒星动力BH质量的星系，也可以对恒星动力方法进行必要的检查。我们从Hobby-Eberly望远镜大规模银河调查中瞄准了87个物体，没有发现任何新的maser磁盘。大多数目标物体是椭圆星系，它们的典型恒星速度色散为250 km s？1，距离在130 Mpc之内。我们讨论了未检测到的含义，无论它们暗示着需要进行批处理的阈值X射线光度，还是可能反映了在低Eddington比下将批处理磁盘保持在更大质量的BH周围的困难。鉴于maser磁盘可用来探测BH增生和人口统计信息，我们建议未来的maser搜索应努力消除残留的样本偏差，以理清这些协变效应的重要性。
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