Extragalactic Radio Sources and Galaxy Formation

我们观察到，我们生活在星系的宇宙中，星系是由数十亿颗恒星组成的大规模有序集合，其中许多恒星都有行星，其中至少有一个星球上有生命。我们生活的“为什么”是哲学、宗教；“它是如何发生的”是科学的领域。我们几个世纪以来的观察使我们得出了目前共识宇宙论的“如何”；但这种共识宇宙论比虚构的更奇怪。“如何”是这样的。在宇宙射电背景上看到的早期宇宙中的微小波动，是早期星系--早期星系--被重力聚集在其周围更大、更杂乱的结构的种子。它们是由恒星照亮的，也是由引力形成的，大爆炸中产生的原始氢和氦气体的丰富燃料坍塌形成了第一批核能发电机。随着恒星轨道进入重力驱动的模式--通常是螺旋形或椭圆形--星系增长并变得有序。引力等级中的星团将这些星系聚集成一种被称为“宇宙网”的设计，这是一个星系团和细丝状星系的网络，其交点-节点-代表星系团。在20世纪末，情况发生了变化，变得非常奇怪。诺贝尔奖获得者对遥远星系中超新星的观测迫使人们做出这样的推断：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，来自尚未构思的几何学和物理学。暗能量占宇宙质量的70%。另有25%存在于“暗物质”中，即我们还无法观测到的弱相互作用粒子。剩下的5%是以我们知道和热爱的物质的形式存在的，我们、地球和我们的可观测星系宇宙都是重子组成的，构成了标准的原子和分子。这就是“LambdaCDM”宇宙学，Lambda描述了未知的加速器，CDM描述了冷暗物质：我们宇宙的95%仍有待理解。在这样的宇宙中形成星系是一个棘手的平衡行动。LambdaCDM宇宙学邀请了宇宙网络中星系的分级重力形成。但如果不加以控制，它就太成功了--我们周围的所有星系，本星系，都应该是蓝色的，形成恒星，而且巨大。取而代之的是，它们通常都很老，“又红又死”，而且很小。有什么东西阻止了星系的形成。这一指控现在被置于射电活动星系核的大门上，射电活动星系核是具有双叶射电源的星系，由来自中央超大质量黑洞(SMBH)的两个相对论光束提供动力。通过测量星系的自转速度，我们知道大多数星系的中心都有一个黑洞，而大型星系则拥有SMBH。这些星系通过双喷流系统释放出的能量可以在这些星系及其邻近星系周围形成巨大的空洞，阻止物质的流入。这样，分级建立过程就停止了；射电活动的短暂阶段(“射电星系”阶段)表明反馈在起作用。直接的X射线观测向我们展示了射电活动星系核周围的这些空洞。我建议结合观测和分析来检查不同类型的射电活动星系核的空间密度，以形成一个关于射电相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？哪些类型的射电星系在这一角色中最有效，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？关键在于反馈的假设是否与现代数据一致。意义何在？在这样的分析完成之前，我们关于星系形成的图景--甚至我们在星系中存在的图景--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.