Black Hole Grand Unification

Active galaxies are one of the most powerful phenomena in the Universe, capable of shaping its evolution. We now know that each active galaxy is ultimately powered by the supermassive black holes, up to a billion times more massive than our Sun, which reside at the heart of every galaxy. But in the case of active galaxies the resident supermassive black hole is being fed by gas from the `host' galaxy to produce powerful emission of light in all wavelengths from its nucleus. The types of Active Galactic Nuclei (AGN) are diverse: they range in power from the relatively faint low-luminosity AGN, to the mighty `quasars' which can outshine the starlight from their entire host galaxy. Also, some AGN show powerful jets of matter emanating from close to the black hole, which cause intense radio emission so that astronomers call these AGN `radio-loud'. Despite many years of study, the origin of these powerful jets and why some AGN show them and other, `radio-quiet' AGN have much weaker jets, remains a mystery. But it is an important mystery to solve, because these jets can interact with the environment surrounding the growing host galaxy to alter its formation, and even stop its growth altogether. Understanding the origin of the AGN jets and why AGN show such diversity, may be crucial to understanding how each galaxy, and the Universe itself, appears the way it does today. Recently, a new paradigm has emerged which throws new light on the diversity of AGN and the origin of jets. This paradigm, which we call `black hole Grand Unification', compares the behaviour of AGN with that of `black hole X-ray binary systems' (BHXRBs) - star systems where a normal star orbits and feeds a relatively small black hole only ten times more massive than our sun. These BHXRBs appear to be a microcosm of AGN behaviour. For example, extensive research I have been involved in has shown that the variations in X-ray light output from AGN are the same as in BHXRBs, except slowed down by a factor of a million or more, as one would expect if the AGN black holes, and the accompanying `accretion disks' which feed them are a million or more times larger than in BHXRBs. Also, comparisons of the amount of radio emission compared to X-ray emission in BHXRBs and AGN shows the same simple scaling relation, which also implies that more massive black holes show more radio emission, and links the X-ray and jet behaviour in a fundamental way for all black holes over a huge range in mass. Crucially, these deep similarities between BHXRBs and AGN point the way to another fundamental connection between these black hole systems, which could explain why some AGN show powerful jets and others don't. BHXRBs also show strong jets, but only in certain `states', which correspond to the rate at which gas is being fed (accreted) to the black hole. At low rates, only the `low/hard' state shows strong jets, is seen. At higher accretion rates the BHXRBs can exist in two states, the `intermediate' state, which probably correspond to rapid changes in accretion rate and has powerful jets, and the more stable high/soft state, which shows no strong jet emission. The black hole Grand Unification paradigm therefore suggests that the radio-loud AGN with strong jets are the supermassive analogues of BHXRBs in the low/hard or intermediate states, while the radio-quiet AGN are the analogues of the high/soft state BHXRBs. In other words, whether an AGN has strong jets or not depends on its accretion rate and the history of changes in accretion. The main aim of my new research is to test this idea, by systematically measuring the properties of BHXRBs in the different states, then scaling them up to what we would expect to see in the much more massive AGN, and testing these predictions against data from large surveys of many AGN, to see if the radio-quiet/radio-loud dichotomy really can be explained within the black hole Grand Unification paradigm.