Properties of the earliest galaxies

The gross cosmological parameters of the Universe that describe how fast it expands and how its expansion changes with time are thought to be known to a fair degree of accuracy. However, we do not have a deep understanding of the material of the Universe - the dark matter and dark energy that dominate over the ordinary ('baryonic') matter that is the familiar stuff of every-day life. We also don't understand the processes that cause this baryonic matter to form into structures such as planets, stars, galaxies, and clusters of galaxies. The purpose of the research to be funded by this grant is to gain some understanding of the largest-scale phenomena that affect the formation of structure by looking at the formation and evolution of galaxies and clusters of galaxies, and the internal substructure that they contain. Clusters of galaxies are often studied by their X-ray emission, which comes from hot gas held by the gravity field of their huge masses. At Bristol we also look at the gas in another way, by the 'shadow' that it casts against the microwave background radiation, which is a universal radiation field that was created soon after the Big Bang. Comparing the results from these ways of finding clusters tells us a lot more about the gas, and so about the mass holding the gas, than either technique alone. This trick is useful for discovering how much mass in the cluster is made up by dark matter and how much is baryonic matter, and whether these components of the mass of the cluster are distributed differently. Such a difference in distribution can occur in the cluster formation process, as it settles into a steady state, or later as the gas radiates energy away. We can also use the clusters that we find to study the expansion of the Universe itself, to find out more about the mysterious dark energy. There is a problem caused by the energy radiated by clusters - as the gas cools, it should drop inwards. But we see too little central gas - something is regulating the infall. It is thought that a major influence on the gas, and perhaps a source of all the energy needed to stop the infall, is the outflow of material from active galaxies, particularly radio galaxies, in the centres of the clusters. Active galaxies are galaxies where there seems to be a very massive black hole at their cores. These black holes themselves have the mass of a small galaxy, and are capable, somehow, of producing flows of gas at close to the speed of light away from themselves. This is a bit strange, since we normally think of black holes as being places where everything falls inwards, and the physics of how the outflows work, and how much energy they produce, is largely unknown. We need to measure that energy, and understand the physics of the process, in order to understand how black holes affect the clusters and galaxies in which they are located, and we will do much work on the radio, X-ray, infra-red properties of galaxy cores, and some theory, to try to understand what is going on. We expect to find out a lot about the black holes themselves, too. The most obvious feature of clusters of galaxies is the galaxies themselves, and we are also interested in knowing how galaxies form and change with time, why there are different types of galaxy, and how the galaxies affect the Universe as a whole. We have found that the stars in the earliest galaxies emit enough radiation to cause the entire Universe to change from being cold to being very hot, so that gas in the Universe changes from being neutral atoms to being a plasma, at a temperature like that of a star. How this happens, and what those first galaxies look like, is a focus of our research. We also want to know what happened to these early galaxies as they collided with one another, as their stars aged (and perhaps exploded), and as their central black holes kept pumping out energy, so we look also at nearby galaxies to study changes over the history of the Universe.