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The Giants of the Universe

by YPU Admin on June 23, 2016, Comments. Tags: Galaxies, Physics, Research, Universe, and UoM


My name is Monique Henson and I’m currently in the second year of my PhD in Astrophysics. I finished my A-levels in Maths, Physics and Further Maths in 2010 and went on to study Physics at the University of Manchester. After my first year, I realised that wanted to focus more on the theoretical aspects of Physics, so I switched to the Physics with Theoretical Physics course. For most of my degree, I wasn’t really sure what I wanted to do afterwards. To help me decide, I did a few different internships during my summer holidays. I tried teaching, working for an international technology firm, and finally I tried academic research.

Before that summer project, I hadn’t thought too much about doing research. If I’m honest, I didn’t realise what a researcher does on a day-to-day basis. I now know that the day-to-day work of a researcher depends a lot on what they are researching! But all researchers are united by one thing - curiosity. Doing that summer project reminded me why I wanted to study Physics in the first place, and made me realise that I wanted to pursue it further.

I started my PhD in 2014. My research involves studying the biggest objects in the Universe that are held together by gravity - galaxy clusters. These giants are made up of thousands of galaxies. Each of those galaxies is made up of hundreds of billions of stars. Some of those stars will be just like our Sun.

In Depth

Why should we study galaxy clusters?

Despite their name, galaxy clusters aren’t just made of galaxies. They also have two other key parts - hot gas and dark matter. Most of the visible mass in galaxy clusters actually exists in between the galaxies. It takes the form of gas that is so hot it emits X-rays. The galaxies around the cluster faster than bullets, and their interaction with this hot gas causes them to rapidly evolve. By studying the galaxies in galaxy clusters, we can learn more about how galaxies change over time.

Most of the mass in clusters is actually dark matter, which is the name we give the substance that makes up most of the mass in the Universe, even though we can’t see it. It doesn’t reflect, emit or absorb light, which means that we can only detect it by looking for its effect on other things. Since galaxy clusters are so massive and around 85% of their mass is in dark matter, then that means they’re great for studying dark matter.

On top of all of that, the number of galaxy clusters in the observable Universe at a given point in time tells us both about how the Universe has expanded over time and how structure forms in the Universe. This technique is called cluster counting as it involves counting the number of clusters with a particular mass within a given volume of the sky.

[The galaxy cluster MACSJ0717. The bright points in the image are galaxies, some of which are in the cluster, whilst others are behind it. The blue-purple material is hot, X-ray emitting gas. If you looked at the cluster with just your eyes then you wouldn’t see it. Instead you need an X-ray telescope, like the Chandra telescope. Credit: NASA, ESA, CXC, C. Ma, H. Ebeling and E. Barrett (University of Hawaii/IfA), et al. and STScI]

What am I trying to find out?

To use cluster counting you have to be able to measure the masses of galaxy clusters really well. It’s quite hard to figure out the mass of something just by looking at it, but there are a couple of different methods that we use. One of these is called gravitational lensing. When light passes by a massive object, such as a galaxy cluster, it can get bent around the object through gravity. When we look at clusters we see that galaxies behind the cluster can look smeared or distorted. This distortion effect is dependent on the mass of the cluster, and by measuring it we can figure out the cluster’s mass.

It’s widely thought that this technique is very accurate for measuring cluster masses. I’m testing this by using this technique on a set of model clusters ran by Dr David Barnes at the University of Manchester.

Going Further

To learn more about galaxy clusters, have a look at the website for the Chandra X-ray telescope. They have some great images of clusters and a blog with regular updates.

One Minute Astronomer has a great article on gravitational lensing here. Gravitational lensing isn’t just used to find out cluster masses; other researchers use it to find planets and to study distant supernovae.

If you’d like to stay updated with my research and outreach activities, follow me on Twitter: @monique_henson