Galaxy clusters are the most massive gravitationally bound structures in the Universe. A galaxy cluster typically contains 50-1000 galaxies of varying sizes and colour – a typical galaxy is made up of 100 billion stars (100,000,000,000). By far the biggest component of a galaxy cluster is the dark matter, which is about 10 times more massive than all the galaxies put together. Being full of galaxies in relative close proximity to each other, and some of the highest concentrations of dark matter in the Universe, galaxy clusters are also the densest regions or matter in the Universe. Here is a nice example of a galaxy cluster, Abell 2218…

Abell 2218
Abell 2218 is a rich galaxy cluster composed of thousands of individual galaxies. It is about 2.1 billion light-years from the Earth (redshift 0.17) in the northern constellation of Draco.

After the Big Bang, in theory all the matter in the Universe should have been evenly distributed in space. If everything is evenly distributed, then everything should exert an equal gravitational force on everything else, in this case no structures should form and the Universe should have remained uniformly featureless. Fortunately for us, the Universe wasn’t full of evenly distributed matter, there were some regions of the early Universe that had slightly more matter than others. These slightly more dense regions slowly gravitated more matter towards them and structures started to form, which would eventually become galaxies, stars and planets. Being the most massive and dense objects observed today means that galaxy clusters were in the densest regions of matter after the Big Bang, and were the first bits of the Universe to start collapsing and forming structures. Studying galaxy clusters helps us learn about why the Universe wasn’t uniform after the Big Bang, and why there is structure today, this also tells us things about the Big Bang itself.

My research in astrophysics was focused on the most massive galaxies in clusters. In the centre of most clusters there is a really massive galaxy, like the big, fat yellow galaxy in the middle of Abell 2218 above. These galaxies are known as ‘brightest cluster galaxies’, or BCGs. Being in the middle of the cluster, studying the evolution of BCGs tells us about the evolution of the cluster as a whole. The centre of a galaxy cluster is a rather extreme place to live – there are frequent and dramatic interactions between galaxies, such as mergers and tidal stripping. This makes BCGs really interesting objects for studying extreme environments and the evolution of galaxies generally.



What did I actually used to do with my time when doing astro?