Extragalactic Radio Sources and Galaxy Formation

我们观察到，我们生活在一个由星系组成的宇宙中，这是一个由数十亿颗恒星组成的巨大有序的集合，其中许多恒星都有行星，至少有一颗行星上有生命。我们活着的“原因”是哲学、宗教；“事情是如何发生的”是科学领域的问题。几个世纪以来的观测使我们对“如何”形成了目前的共识宇宙学；但这种一致的宇宙论比小说更奇怪。“怎么做”是这样的。我们早期宇宙的微小波动，可以在宇宙射电背景中看到，是在引力作用下聚集更大、更混乱结构的种子——早期星系。它们是由恒星点亮的，也是由引力形成的，大爆炸中产生的丰富的原始氢和氦气燃料坍塌形成了第一个核能发电机。随着恒星轨道进入重力驱动模式（通常是螺旋形或椭圆形），星系不断增长并变得有序。这些星系在引力层次上的聚集形成了一个被称为“宇宙网”的设计，这是一个星系团和细丝中的星系网络，它们的交叉点——节点——代表了星系团的星系团。在20世纪末，情况变得非常奇怪。诺贝尔奖得主对遥远星系中超新星的观测迫使人们推断出，我们的宇宙正在被一种神秘的“暗能量”加速。它的起源要么是简单的，作为广义相对论的可能结果，要么是复杂的，来自尚未被构想出来的几何学和物理学。暗能量占宇宙总质量的70%。另外25%存在于“暗物质”中，即我们还无法观察到的弱相互作用粒子。剩下的5%是以我们所了解和喜爱的物质的形式存在的，而我们、地球和宇宙中可观测的星系都是由这些物质组成的——重子，它们构成了标准的原子和分子。**这是“LambdaCDM”宇宙学，Lambda描述未知的促进剂，CDM描述冷暗物质：我们宇宙的95%仍有待了解。在这样一个宇宙中星系的形成是一个棘手的平衡行为。LambdaCDM宇宙学提出了宇宙网络中星系的分层重力形成。但如果不加以控制，它就太成功了——我们周围的所有星系，本地星系，应该是蓝色的，形成恒星，而且很大。相反，它们通常是老的、“红的、死的”、小的。有什么东西阻止了星系的形成。这些电荷现在被放置在射电活动星系核（AGN）的门口，该星系拥有双叶射电源，由来自中央超大质量黑洞（SMBH）的双相对论光束供电。通过测量星系的自旋，我们知道大多数星系的中心都有一个黑洞，而大型星系中也有SMBHs。这些星系通过它们的双喷射系统喷射出的能量可以在这些星系及其邻近星系周围雕刻出巨大的空洞，阻止物质的流入。这样，等级制度的建立过程就停止了；放射性活动的短暂阶段（“射电星系”阶段）表明反馈在起作用。直接x射线观测向我们展示了无线电AGN周围的这些空腔。**我建议结合观察和分析来检查不同类型的射电AGN的空间密度，以形成射电相位如何表现自身的连贯方案。这些数字是否足以产生必要的反馈？哪种类型的射电星系在这个角色中最有效，我们能否将它们的物理特性与反馈过程所需的物理特性联系起来？关键在于反馈假说是否与现代数据相符。意义吗?在进行这样的分析之前，我们对星系形成的了解——以及我们在星系中的存在——仍然是不完整的。

We observe that we live, and we live in a universe of galaxies, massive ordered collections of billions of stars, many of which have planets, with life on one at least. The "why" we live is philosophy, religion; the "how it happened" is the realm of science. Our centuries of observation have led us to a current consensus cosmology of the "how"; but this consensus cosmology is stranger than fiction. **The "how" goes like this. Small fluctuations in our early Universe, seen imprinted on the Cosmic Radio Background, were the seeds around which larger and messy structures were collected by gravity - the early galaxies. These were lit up by stars, also formed by gravity, the copious fuel of primordial hydrogen and helium gas created in the Big Bang collapsing in to form the first nuclear-power generators. The galaxies grew and became ordered as stellar orbits settled into gravity-driven patterns: generally spirals or ellipticals. Clustering in gravity hierarchy settled these galaxies into a design called the "Cosmic Web", a network of galaxies in clusters and in thin filaments whose intersections - nodes - represent clusters of clusters. **At the end of the 20th century the picture changed to become dramatically strange. Nobel-prize winning observations of supernovae in distant galaxies forced the deduction that our Universe is being accelerated by a mystical "Dark Energy". Its origins are either simple, as a possible outcome of General Relativity, or complex, from geometry and physics not yet conceived. Dark Energy constitutes 70% of the mass of the Universe. Another 25% is in "Dark Matter", weakly-interacting particles that we cannot yet observe. The remaining 5% is in the form of matter that we know and love, and of which we, the Earth and our Universe of observable galaxies are all made - baryons, constituting standard atoms and molecules.**This is the "LambdaCDM" cosmology, Lambda describing the unknown accelerant, CDM the Cold Dark Matter: 95% of our Universe remains to be understood.**Galaxy formation in such a universe is a tricky balancing act. The LambdaCDM cosmology invites hierarchical gravity formation of galaxies in the cosmic web. But left unchecked it is too successful - and all the galaxies around us, the local galaxies, should be blue, forming stars, and huge. Instead they are generally old, "red and dead", and small. Something stops galaxy formation in its tracks. That charge is now laid at the door of radio AGN, radio Active Galactic Nuclei, galaxies with double-lobed radio sources powered by twin relativistic beams from a central super-massive black hole (SMBH). We know from measuring the spin of galaxies that most have a black hole at the centre, and that large galaxies harbour SMBHs. The energy these blast out via their twin-jet systems can carve giant cavities around these galaxies and their neighbours, stopping the infall of matter. In this way the hierarchical build-up process is halted; the brief phase of radio-activity (the "radio galaxy" phase) is indicative of feedback in action. Direct X-ray observations show us these cavities around radio AGN. **I propose a combination of observations and analysis to examine the space densities of different types of radio AGN, to form a coherent scheme of how the radio phase manifests itself. Are the numbers adequate to produce the necessary feedback? What types of radio galaxy are most effective in this role, and can we relate their physics to that required by the feedback process? What is at stake is whether the hypothesis of feedback is consistent with modern data. The significance? Until such an analysis is carried through, our picture of galaxy formation - and indeed of our existence in galaxies - remains incomplete.