Understanding the symbiosis between galaxy evolution and AGN activity

活动星系拥有异常明亮的类星核，这不能用组成星系晕的数千亿颗恒星的正常热发射来解释。然而，这种活跃的星系核(AGN)是由于星系中心超大质量黑洞吸积的气体加热所致，这是整个星系演化过程的一部分。最近的一项重大进展表明，超大质量黑洞的质量--通常在太阳质量的1000万零10亿倍之间--与宿主星系的质量密切相关。有人认为，这种密切的关系可以用活动星系核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，它们会驱动强大的外流，最终将气体从星系中排出，阻止活动星系核的活动和星系晕中任何进一步的恒星形成。活动星系核反馈效应也有助于解释为什么在局部宇宙中高质量星系相对较少：活动星系核外流通过排出原本会形成恒星的冷却气体来防止星系变得太大。然而，尽管活动星系核反馈效应对理解星系的演化具有潜在的关键意义，但它在很大程度上是一个理论构造，在直接观测局部宇宙中的活动星系方面还没有坚实的基础。随着星系的演化，触发活动星系核活动所需的气体如何以及何时积累也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，基于谢菲尔德活动星系核小组超过15年的研究经验，测量附近各种活动星系中活动星系核外流的功率，并研究主要的活动星系核触发机制。这将使我们能够评估活动星系核对于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.