Understanding the symbiosis between galaxy evolution and AGN activity

活跃的星系有异常明亮的星状核，这不能用构成星系晕的数千亿颗恒星的正常热辐射来解释。尽管如此，这种活跃的星系核（AGN）是星系中心超大质量黑洞吸积气体加热的结果，是整个星系演化过程的一部分。最近有一项重大进展表明，超大质量黑洞的质量——通常是太阳质量的100万到10亿倍——与宿主星系的质量密切相关。有人提出，这种密切的关系可以用AGN的负反馈效应来解释：随着超大质量黑洞通过气体的吸积而增长，它们驱动强大的外流，最终将气体从星系中驱逐出去，阻止AGN的活动和星系晕中任何进一步的恒星形成。AGN反馈效应也可以帮助解释为什么在局部宇宙中相对较少的高质量星系：AGN流出物通过排出冷却气体来防止星系变得太大，否则会形成恒星。然而，尽管AGN反馈效应对理解星系演化具有潜在的关键意义，但它在很大程度上是一个理论建构，在对局部宇宙中活跃星系的直接观测中还没有坚实的基础。随着星系的演化，触发AGN活动所需的气体是如何以及何时被吸积的，仍然存在很大的不确定性。因此，我们建议使用最先进的技术，基于谢菲尔德AGN小组超过15年的研究经验，测量各种附近活动星系中AGN流出的功率，并研究主要的AGN触发机制。这将使我们能够评估AGN对我们对星系演化的一般理解的真正意义。

Active galaxies have unusually bright star-like nuclei which cannot be explained in terms of the normal thermal emission of the hundreds of billions of stars that make up the galaxy haloes. It is though that such active galactic nuclei (AGN) result from the heating of gas accreted by super-massive black holes in the centres of the galaxies as part of the overall galaxy evolution process. In a major advance it has recently been shown that the masses of the super-massive black holes -- typically between a million and a billion times the mass of our Sun -- are closely related to the masses of the host galaxies. It has been suggested that this close relationship can be explained in terms of the negative feedback effect of the AGN: as the super-massive black holes grow through the accretion of gas they drive powerful outflows that eventually expel the gas from galaxies, halting the AGN activity and any further star formation in the galaxy haloes. The AGN feedback effect can also help to explain why there are relatively few high mass galaxies in the local Universe: the AGN outflows prevent galaxies from growing too massive by expelling the cooling gas that would otherwise form stars. However, despite its potentially key importance for understanding the evolution of galaxies, the AGN feedback effect is largely a theoretical construct, and does not yet have a solid basis in direct observations of active galaxies in the local Universe. There also remain large uncertainties about how and when the gas required to trigger the AGN activity is accreted as the galaxies evolve. Therefore we propose to use state-of-the-art techniques, based on the over 15 years research experience of the AGN group in Sheffield, to measure the powers of the AGN outflows in a variety of nearby active galaxies, and also investigate the dominant AGN triggering mechanism. This will allow us to assess the true significance of AGN for our general understanding of the galaxy evolution.