The distribution of black hole growth across the evolving galaxy population

Understanding how galaxies form and evolve over time is one of the most important and complicated challenges in astrophysics. At the centre of every galaxy (including our own galaxy, the Milky Way) there is thought to be a massive black hole that is a million to a billion times the mass of the Sun. It is now thought that these massive black holes could determine much of the structure and evolution of the galaxies which host them, although exactly how and why is still unclear.

My research focuses on these important connections between galaxies and black holes. In particular, I study galaxies where the black hole is rapidly growing as matter falls in. Before falling in, this matter gets hot, emitting huge amounts of radiation over the entire electromagnetic spectrum. A galaxy where the black hole is growing in this way is described as having an active galactic nucleus. I aim to understand why some galaxies have these active nuclei, whereas in other galaxies (such as the Milky Way) the black hole is not rapidly growing. Some mechanisms must drive material into the central regions of a galaxy and fuel this black hole growth, but we are not yet sure what these mechanisms are. Furthermore, the huge amounts of energy released as the black hole grows could have a significant impact on the rest of the galaxy, heating up or ejecting gas and preventing the formation of new stars. Thus, determining when black hole growth occurs within the lifetimes of galaxies is vital to understand the physics of galaxy evolution.

My recent work has shown that active nuclei (powered by growing black holes) may flicker on and off over timescales of a few hundred thousand years, which is relatively short compared to the lifetime of a galaxy. This flickering makes it much harder to link the overall levels of black hole growth to the evolution of the galaxy population. To make progress, we need to measure the *distribution* of black hole growth rates over large samples of galaxies in similar evolutionary phases. Mapping the distribution of black hole growth in such detail has, until now, been difficult due to the limited size of galaxy samples and the lack of sensitive X-ray observations that are needed to peer into the centres of galaxies and measure the current growth rate of the central black hole.

Over the next few years, this situation will change dramatically. A new space telescope, eROSITA, will map the entire sky at X-ray wavelengths, which I will use to track black hole growth rates within nearby galaxies in unprecedented detail. These studies are particularly important, as the impact of black holes appears to be most severe in the biggest galaxies in the nearby Universe. These galaxies are generally round in shape, red in colour, and tend not to be forming any new stars - possibly all due to the black hole at the centre.

In addition, new surveys are underway with our largest ground-based telescopes, measuring the spectrum of light from large samples of extremely faint, distant galaxies and thus revealing the evolutionary lifecycles of galaxies when the Universe was less than half its current age. I will combine these new faint galaxy surveys with the highest quality X-ray data (from NASA's Chandra telescope) to track black hole growth during this important early epoch of cosmic time.

The combination of these studies will provide a comprehensive picture of the physical mechanisms that drive black hole growth and the impact of black holes on the evolution of galaxies, spanning from the early Universe to recent cosmic times.