The evolution of galaxy components and the influence of environment

Looking out into the universe, far beyond our nearby stars, we find a vast population of galaxies. Most of these are isolated, appearing as islands containing many stars and surrounded by mostly empty space, but they tend to group together due to the influence of gravity. Astronomers refer to various properties describing a particular region of space, including the density of galaxies, as the 'environment'. Galaxies have a wide range of properties, such as their environment, size, brightness, colour, and shape (also known as morphology). In many ways, though, they display strong similarities, and can be categorised. One endeavour of astronomy is to study the distributions of galaxy properties, determine which characteristics are important, and use these to understand how galaxies form and evolve over time. One of the most striking distinctions is between galaxies which have a disky, spiral morphology and those which appear smooth and elliptical. Disk galaxies are usually blue, while ellipticals are red. From our knowledge of nearby stars this suggests that disks contain young stars, but that ellipticals are almost entirely old. However, the division between disks and ellipticals, or blue and red galaxies, is not clear-cut. For example, there exists a significant population of red disk galaxies, especially in dense regions of the Universe. Furthermore, most disk galaxies actually contain a significant central component, called a bulge, with properties similar to a small elliptical galaxy. These different galaxy components can be partly explained by models of galaxy formation, whereby disks are formed from new gas falling onto galaxies, while spheroids (bulges and ellipticals) are created by galaxies crashing into one another. One therefore expects them to be fundamental to describing and understanding the galaxy population. However, work to date has largely ignored this and studied the overall properties of galaxies. This has generally been deemed necessary due to various difficulties in determining morphologies and separating the properties of the two components for sufficiently large numbers of galaxies. Now we have an exciting opportunity to directly access the information in the separate galaxy components, thanks to several developments. Firstly, computational tools are at a stage where they can potentially separate galaxy components quickly and reliably. Secondly, the era of citizen science has arrived: through the Galaxy Zoo project we can now harness the brain-power of large numbers of people to study the features that computers still struggle with. Finally, the latest surveys provide the quantity and quality of data required to study the internal components of galaxies and their environments in a reliable and statistically sound manner. I will analyse a wide variety of data from a number of surveys, measuring the properties of the separate components for a hundred thousand galaxies, and determining morphologies for millions of objects, both nearby and distant. Through this I aim to gain a revolutionary new understanding of the evolution of galaxies in terms of their fundamental internal structures.