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FEEDBACK explored the impact luminous accreting black holes have on their surroundings, particularly on how a massive black hole at the centre of a galaxy can determine the evolution of its gas and thus of the galaxy. From X-ray observations we studied how accretion liberates gravitational energy, in other words how quasars work. They are the most persistent luminous objects in the Universe and most galaxies would have passed through a stage when there was a quasar at its centre. Despite a 100 millionfold difference in physical size, radiation from the black hole can push interstellar gas out of the galaxy, terminating star formation and thus further growth of the galaxy.
We used several defining observations to make progress. In February 2016 the Hitomi satellite make deep X-ray observations of the cool core of the Perseus cluster of galaxies. The resulting spectra were of high resolution unprecedented in cosmic X-ray astronomy, yielding an uncertainty of 10 km/s or less on gas motions. This enabled the levl of turbulence in the cluster core to be determined at only 164 km/s, which means that the energy in turbulence is only 4% of the thermal energy of the gas. The emission of the radiation we detect means that the gas is losing energy and since no massive cooling flow is occurring there must be a balancing source of heat. This is due to the central Active Galactic Nucleus (AGN) pumping out jets seen in the radio band. The jets blow bubbles in the hot intracluster gas and energy somehow goes from there to the rest of the core gas, stemming rapid cooling. A popular previous mode of heating invoked turbulence but the Hitomi results preclude that. We have proposed that the propagation of sound waves transports energy rapidly with only a small displacement velocity.
In summer 2016 a 1.5 Ms long XMM observation of IRAS13224-3809 was made for us. This is a low mass AGN which shows the greatest persistent level of variability, making it ideal for spectral timing studies. We studied the data in great detail, revealing a highly-spinning 2 million Solar mass black hole with a hot corona moving up and down from about 5 to 15 gravitational radii. Variable absorption lines were seen with a blue shift corresponding to about 0. 25c. This could represent a massive wind from the accretion flow, as expected from some AGN feedback theories and past observations. Alternatively, we have shown that it could originate in the accretion flow itself very close to the black hole. now outflow need occur.
We studied radiation-pressure driven outflows from luminous AGN. If the radiation is trapped when the gas is at small radii (<50 pc) then it is possible to drive cold shells of dusty gas to kpc radii at final velocities above 1000 km/s. Such a scenario is consistent with observations in the submm to optical bands of some low redshift AGN. It also explains an avoidance zone in the column density - Eddington fraction plane.
We explored the possibility of electron-positron pair production in the hot corona of accreting black holes. The compactness (luminosity to radius ratio) of such objects shows a relationship with coronal temperature that avoids values where pair production would runaway. This indicates that pair production is involved and that the corona is probably mostly composed of pairs.
We have measured molecular flows in cool core clusters and explored how such massive gas flows move and behave around the central galaxy. The imprints of AGN feedback were also found within a supermassive black hole's sphere of influence. with Chandra X-ray mapping of the hot gas around the nucleus of M87. Gas appears to be cooling as well as inflowing. HST imaging of the spectacular system of filaments at the centre of the Centaurus cluster was obtained and analysed.