Supermassive black hole growth - a small-scale solution to a large-scale problem

In the centre of almost every galaxy sits a dark object, with a mass over a million times that of the Sun. Although massive, these objects have been compressed by their own gravity to extremely high densities, so dense in-fact that light cannot escape their surface; these are the supermassive black holes (SMBHs). How these exotic objects formed is one of the outstanding mysteries facing astronomers and one I find particularly intriguing.

By looking to great distances we are able to glimpse what happened in the first few billion years of the Universe's life. Although very difficult to perform studies at these distances, a few SMBHs can be 'weighed'. Surprisingly this has shown that they were fully-grown when the Universe was only ~1 billion years old. Although scientists thought that growth should take a long time, this discovery demands that they instead grew incredibly quickly by material falling onto the black hole at extreme rates - faster than should be allowed by the balance of radiation and gravity, the so-called Eddington limit. How accretion operates at such rates is unclear but is a pressing issue in astrophysics - an issue I aim to address.

Although we cannot study the SMBHs growing directly, we can observe how material falls onto SMBHs in nearby galaxies and onto much smaller black holes (with orbiting stars in binary systems: BHBs) within our own Galaxy. As a result, we know that material doesn't fall directly onto the black hole but forms an accretion disc, which emits large amounts of radiation. At very high rates, not all of the material falls onto the black hole; instead some fraction is expelled in 'winds' or 'jets'. Winds carry material from the surface of the disc at fairly 'slow' speeds (~10% of the speed of light) whereas jets are much more powerful ejections of matter from close to the black hole at almost the speed of light. It is logical that similar outflows will have accompanied the SMBH growth, with matter taken from the accretion flow and redistributed to the surroundings - a form of 'feedback'.

Understanding the nature of Eddington accretion and the associated outflows is necessary for understanding the growth of SMBHs and the impact feedback must have had on the host galaxy. In practice this requires observing how the accretion flow changes as it reaches the Eddington limit and couples to the outflow, i.e. how they interact. In practice this has proven to be extremely difficult: emission from the accretion flow onto both SMBHs in nearby galaxies and onto Galactic BHBs is obscured by intervening material, preventing a view of the coupling. My proposal approaches this problem in a new way: by looking at BHBs accreting at high rates in nearby galaxies where the amount of intervening material is much lower, allowing the emission from the inflow to be studied. These extragalactic BHBs come in two 'flavours': those which are commonly seen in the Milky Way and show powerful jets, and those which are even brighter and thought to be a more extreme form of Eddington accretion with powerful winds.

I am leading the first major search for new BHBs with powerful jets in two nearby galaxies. By observing with several instruments across a range of wavelengths, including the world's foremost radio telescope, the VLA, and NASA's X-ray satellite, Swift, I will observe how the disc and jets change together, thereby constraining both the nature of the inflow and how the jets are launched. In order to understand the brightest sources with powerful winds, I will combine novel analysis techniques with theory to reveal the nature of both inflow and outflow. By studying accretion onto SMBHs in the local Universe, I will extrapolate my findings to larger black hole masses where the coupling of inflow and outflow cannot be studied. Finally, by using simulations of high redshift SMBH growth I will be able to explore the impact of feedback on the host galaxies, in an epoch otherwise hidden from view.