Cosmic giants on cosmic scales: how to measure the Universe

Our planets orbit the Sun, and our solar system (as well as many others) orbit the centre of our galaxy, also known as the ‘Milky Way’. But did you know that galaxies also orbit around each other? We call these systems galaxy groups and galaxy clusters, and they form some of the largest structures in the observable Universe.

Mass is the fundamental property and is critical to understanding both the composition of the Universe and the cosmological parameters that define it. A typical cluster is 1015 times heavier than our sun. However, the stars and the galaxies which are directly observable (also known as baryonic matter) constitute only a few per cent of their total mass.

In X-ray wavelengths a hot gas is also observable, but this only accounts for around 10 per cent of the mass. Over 85 per cent is what astronomers call dark matter; it has not yet been directly observed but we know it’s there from its gravitational influence.

Galaxy clusters have so much mass that light emitted from distant galaxies travelling in the vicinity of a cluster will be pulled by its gravity, distorting its appearance. This is known as gravitational lensing. In some cases the light is distorted so much that it creates giant arcs on the sky, but in the majority of cases the effect is too small to see and can only be determined by statistically averaging over the shapes and orientations of many galaxies behind the cluster. The more mass a cluster has, the more distorted the background galaxies will be.

With the help of gravitational lensing, I have been able to measure the masses of several galaxy groups and clusters. However, the amplitude of the distortion is only about 1 per cent and it gets increasingly smaller for lower mass systems such as groups and poor clusters. We need to measure the average over a lot of background galaxies in order to obtain accurate cluster masses and that means we need deeper observations of the sky. However telescope time is expensive. Observables such as X-ray temperature and luminosity are a lot cheaper to obtain as they don’t require as much exposure time. By finding relationships between gravitational lensing mass and these cheaper observables, it is easier to determine masses of many more groups and clusters.

There are also many other ways to determine the total mass of a cluster. Assuming that a cluster is in hydrostatic equilibrium (that its radiation pressure is able to support its collapse due to gravity), then the mass can be determined from the pressure and temperature gradients. This is known as hydrostatic mass. Another method is to calculate the virial mass from the velocities at which the galaxies orbit the centre of the galaxy cluster. These masses may not necessarily be equivalent due to the assumptions that go into them. However, by comparing them we can test both the validity of the assumptions and our understanding of physics.

Galaxy groups and clusters make the perfect environment to carry out new research, since their matter composition is said to be representative of the Universe as a whole.

This new study focuses on the development of optimal methods to determine their masses both efficiently and accurately. It will become important for the next generation of astronomy missions, such as e-Rosita.

It will also be important for an X-ray telescope that is predicted to uncover around 100,000 groups and clusters, as well as optical telescopes such as Euclid and LSST. The telescopes will allow us to make more accurate shape measurements of over a billion galaxies generating around 1,000TB of data every day.  

Maggie Lieu

PhD Astrophysics, School of Physics and Astronomy