Extragalactic Radio Sources and Galaxy Formation

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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.