Connecting the lifecycles of galaxies and their central black holes

Understanding why galaxies - including our own galaxy, the Milky Way - look the way they do is a vital part of our mission to understand the Universe around us. At the centre of most, if not all, galaxies (including the Milky Way) lies a supermassive black hole, with a mass that is millions to billions of times the mass of the Sun. These black holes appear to be crucial component of galaxies, determining much of their structure and evolution, although how and why is still unclear. As a Future Leaders Fellow, my team and I are determining how these black holes grow, when this growth occurs within the lifecycle of a galaxy, and how these black holes affect the galaxies they lie in.

Galaxies have long lives. They evolve slowly as the gas within them forms stars, these stars age over billions of years, and the overall shape and structure of a galaxy gradually changes. By comparison, the massive black holes at their centres appear to grow relatively rapidly, in short-lived "growth spurts" lasting at most a few million years. During these growth spurts, material falls towards the black hole, heats up and can produce huge amounts of radiation spanning the entire electromagnetic spectrum. A galaxy with such a growing black hole is described as having an "active galactic nucleus" or AGN. Over recent years, astronomers have started to understand the structure of the material close to a black hole that forms an AGN and how these structures produce radiation at different wavelengths - including X-rays, optical and infrared light and radio emission. These structures result in a wide range of observed phenomena in different galaxies, all of which can be associated with an AGN powered by a growing supermassive black hole. However, we do not know how these structures change over millions of years or longer for an individual AGN, over timescales relevant for galaxy evolution.

My team and I are developing new techniques to connect the lifecycles of galaxies and their central black holes, probing timescales of millions to billions of years. Our studies reveal how material is brought into the centre of a galaxy, how this material forms an AGN, how the AGN changes and eventually fades over the course of many millions of years, what impact the AGN has on the galaxy during this time, and how often this whole process repeats throughout the lifetime of a galaxy.

Studying processes on such long timescales is extremely challenging. We cannot watch a single galaxy over this time, so instead, we are using new surveys of the sky that provide "snapshots" of many millions of galaxies and their AGN, each at a different life stage. We have developed new methods to extract information from these static snapshots and are building new models to describe the lifecycles of individual AGN and explain what we see in these snapshots. We are also studying the properties of galaxies with (and without) AGN to determine whether certain types of galaxies are more likely to have an AGN. In doing so, we can determine which physical mechanisms within galaxies are responsible for driving material into their central regions and fuelling the periods of AGN activity. Our work combines data from an array of new astronomical surveys. We are using new surveys that are underway with the premier ground-based telescopes, measuring the spectrum of light from large samples of galaxies that is imprinted with vital information on their lifecycles, combined with new X-ray imaging of the sky provided by the recently launched eROSITA telescope as well as existing infrared data and new surveys at radio wavelengths that can be combined to track the variation in AGN structure within these galaxies.