Ionised outflows: the missing link in AGN-host galaxy evolution?

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At the heart of most galaxies lies a gigantic black hole with a mass of up to a thousand million Suns. In some nearby galaxies, the gravitational pull of the black hole draws in matter from a rotating disc of gas and dust that surrounds it; before this material is swallowed forever, it loses energy by emitting radiation including visible light, ultraviolet and X-rays. The total energy output of this so-called active galactic nucleus can be greater than that of the whole galaxy that it resides in. Clouds of gas stream away from the nucleus at speeds of several hundred to several thousand kilometres per second. The gas clouds are highly ionised (most of the electrons have been stripped from the atoms) and originate in material blasted out of the black holes's environment by the intense radiation field. It is this radiation which causes the ionisation of the gas. These outflows we see in nearby galaxies could be just the ghostly reminder of a far more dramatic phenomenon at an earlier, violent era in galactic evolution. Increasing attention is now being focused on how ionised outflows connect the evolution of the black hole to that of the surrounding galaxy. While the black hole grows, it is shrouded in gas and dust, obscured from sight. Once the black hole is big enough, the radiation output from accretion can blow away this material, and also matter in the surrounding galaxy that is forming stars. This massive outflow thus ends the process of star formation in the galaxy. Over time, as more and more matter is either consumed by the black hole or driven away in the outflow, the supply of accretion fuel is depleted until the black hole becomes dormant. Ionised outflows from active galactic nuclei are thus central to the development of galaxies and the giant black holes at their cores, and therefore to cosmic evolution as a whole. We study them through the effect they have on the radiation emitted from the active nucleus; radiation at certain energies is absorbed away by the outflowing gas, and by observing the patterns of these absorption features with spectrometers, we can tell how fast the gas is moving, how much gas lies between us and the black hole, and how highly ionised it is. X-ray and ultraviolet radiation from space is mostly prevented from reaching us on the Earth's surface since, fortunately for us, it is absorbed by the atmosphere. We therefore have to study the high energy radiation from deep space objects using telescopes on board satellites in orbit around the Earth, above the screening effects of the atmosphere. My research involves studying, through X-ray spectroscopy with space telescopes, the physical properties of ionised outflows both in nearby and in distant active galaxies (because of the time light takes to reach us, more distant objects probe earlier times in the history of the universe). The ultimate aim is to provide factual information with which to test theories of how the outflow connects the evolution of the black hole and the host galaxy. The key factor in this is determining the rate at which mass is flowing out of the nucleus and into the surrounding galaxy; I will estimate this using physical properties measured from absorption features in X-ray and ultraviolet light. I aim to estimate the mass outflow rates from spectra of active galactic nuclei at a wide range of distances from us, in order to see how the mass outflow rates change over cosmic time. I will then compare the results of this with the history of star formation and the lifecycles of black holes, in order to understand how the ionised outflows connect the evolution of the two.