WHAT I REALLY WANT TO KNOW IS…
Caroline Foster is studying stellar motions in hundreds of galaxies to determine how their structures form and evolve
What gives a galaxy its shape?
The Universe contains billions of galaxies, which to us look like flat images projected on a screen. When Edwin Hubble proved that these faint blurs were separate galaxies beyond our Milky Way a century ago, he wondered about their true shapes. Astronomers have pondered this problem for decades but have been unable to find answers. It is important that we do, because knowing a galaxy’s shape can help us to learn how it formed and evolved.
Some galaxies are like flat discs, others like squashed rugby balls. We think that gas falling into a galaxy to form new stars leads to a flatter disc, while galaxies which collide and merge produce more spherical structures.
Looking at the motions of stars within galaxies can help us to infer what their form is. That has been recognised for a long time, but the necessary data was not there. I realised that the latest technology now allows us to collect this data, and to do so from large numbers of galaxies. This gives me a large enough sample to get an idea of how galaxies are shaped according to factors such as how fast they rotate.
Going three dimensional
To reveal the shapes of galaxies, I measure the motions of the stars. That gives me the full dynamics of the stars in 2D, so I can see how different groups of stars move within a galaxy. When I have that data for the many positions in one image, I can see how the whole galaxy is rotating, and that helps me to picture the galaxy in three dimensions.
My work on this project is carried out at the Australian Astronomical Observatory, in New South Wales. The Anglo-Australian Telescope, with its 3.9m mirror, has a clever piece of kit attached called SAMI – the Sydney Australian Astronomical Observatory Multi-Object Integral Field Spectrograph). It is able to obtain a spectrum for each point in an image it takes of the sky, but it can do that for 13 galaxies at a time. Being able to measure so many galaxies in one go is really key to the project’s success. It made me realise that SAMI could revolutionise this field of research. For the first results reported from this project, we observed 845 galaxies, but we had collected more than 1,000 measurements. They don’t all have the right signal-to-noise quality that we need, so we had to reject some. So what have I found so far? We know that galaxies merge, and we’ve seen mergers on the images. There are some simulations that show us how mergers can reorganise the orbits of stars, and if the orbits of stars are reorganised then that is going to change the shapes intrinsically of those galaxies. These merging galaxies typically don’t have fuel for further star formation, so we just see a reorganising of the present stars’ orbits. But other galaxies still have a lot of gas in or around them, and that gas can collapse and create new stars, and that will change its shape as well. It can even reform a disc. So depending on the different things that come into play, you expect to have different shapes. My first results show that the obvious thing to affect a galaxy’s shape is its rate of spin. If it spins very fast, then it will become very flattened and produce a quite circular disc. On the other hand, galaxies that rotate very little, or not at all, have more varied shapes, like squashed balls or sea urchins. I plan to build on this study by seeing how a galaxy’s local environment affects its shape, and how shapes change with age. Such research should also help inform us about such things as the dark matter around galaxies.