Why are only some supermassive black holes actively feeding as AGN?

在大多数（如果不是全部的话）大质量星系的中心都有一个超大质量黑洞（SMBH; Ferrarese & Ford, 2005，《空间科学》）。Rev.116, 523)。SMBHs通过自我调节的物质吸积而成长，在此期间，它们以活动星系核（AGN）的形式存在，并释放出大量的辐射能和机械能。这种能量通过驱动气体外流来影响它们的宿主星系，也通过加热周围的星系间或星系团内介质来影响它们更大的环境。然而，SMBHs的普遍存在与AGN的缺乏形成了鲜明对比。不到10%的大质量星系是AGN (Martini et al., 2013, ApJ, 768,1)，这意味着SMBHs在其生命的大部分时间里都处于休眠阶段。这就提出了一个难题，因为大多数大质量星系都有现成的热气体或冷气体供应，可以为SMBH提供燃料。考虑到SMBH与星系生长之间的联系（Ferrarese & Ford, 2005），以及AGN反馈对星系演化的破坏性影响（Bower et al. 2012， MNRAS, 422, 2816），我们对触发AGN的原因缺乏了解是银河系天体物理学中一个关键的未解决问题。AGN的环境为各种AGN触发器提供了基础测试（Haines et al., 2012; ApJ, 754,97）。例如，如果核活动需要低温气体或星系合并，那么AGN应该主要分布在低质量群和细丝中，这些细丝会形成星团。另一方面，如果核活动是由高温气体驱动的，我们预计AGN将在群体和集群的核心区域最为普遍。如果AGN的活动是一种随机现象，与环境无关，那么我们就可以期望AGN的分布反映出恒星形成星系（它们最常见的宿主星系）的分布。到目前为止，由于获得光谱和识别AGN的费用很大，对AGN环境的研究范围有限。为了解决这一数据不足的问题，将于2020年在拉帕尔马的4.2米威廉·赫歇尔望远镜上安装1000根纤维馈送的多目标光谱仪（WEAVE）。即将到来的由英国领导的WEAVE-LOFAR （W-L）调查将使用该仪器收集包括AGN在内的100多万个活动星系的光谱。在本项目中，您将使用W-L来确定不同类型AGN的精确环境历史。

At the heart of most, if not all, massive galaxies lies a supermassive black hole (SMBH; Ferrarese & Ford, 2005, Space Sci. Rev.116, 523). SMBHs grow through the self-regulated accretion of material, during which time they present as active galactic nuclei (AGN) and release enormous amounts of radiative and mechanical energy. This energy impacts both their host galaxy, by driving gaseous outflows, and their larger environment through heating the surrounding intergalactic or intracluster medium. The ubiquity of SMBHs, however, contrasts with the paucity of AGN. Less than 10% of massive galaxies are AGN (Martini et al., 2013, ApJ, 768, 1), meaning that SMBHs lie in a dormant phase for most of their lifetime. This presents a conundrum as most massive galaxies have a ready supply of hot or cold gas which could feed the SMBH. Given the link between SMBH and galaxy growth (Ferrarese & Ford, 2005), as well as the devastating impact of AGN feedback on galaxy evolution (Bower et al. 2012, MNRAS, 422, 2816), our lack of understanding of what triggers AGN is a key unresolved issue in galactic astrophysics.The environment of AGN provides a fundamental test for a variety of AGN triggers (Haines et al., 2012; ApJ, 754,97). For example, if cold gas or galaxy mergers are required for nuclear activity then AGN should be distributed primarily in low mass groups and the filaments that feed into clusters. On the other hand, if nuclear activity is fuelled by hot gas we would expect AGN to be most prevalent in the core regions of groups and clusters. If AGN activity is a stochastic phenomenon, unrelated to environment, then we would expect the distribution of AGN to mirror that of star forming galaxies (their most common host galaxy). So far, the environment of AGN has been investigated in limited scope due to the expense of obtaining spectra and identifying AGN over wide areas. To solve this lack of data, the 1000-fibre-fed multi-object spectrograph (WEAVE) will be placed on the 4.2m William Herschel Telescope in La Palma in 2020. The forthcoming UK-led WEAVE-LOFAR (W-L) survey will use this instrument to gather more than a million spectra of active galaxies, including AGN. In this project you will use W-L to identify the precise environmental history of different types of AGN.