Colliding galaxies

The 100 billion galaxies in the universe are not just scattered randomly in space. Most galaxies are arranged in small groups or larger clusters.

What is a galaxy cluster?

Galaxy clusters are the largest structures in the universe that are held together by gravity.

A single galaxy has millions or billions of stars, with clouds of dust and gas, all held together by gravity. A galaxy cluster is a large group of between 200 and 1000 galaxies, attracted together by the gravity of a huge central galaxy. Not all galaxies are part of galaxy clusters – our own Milky Way is part of a smaller local group of galaxies.

Studying galaxy clusters

You’d think it would be easy to study the largest structures in the universe. Dr Melanie Johnston-Hollitt at Victoria University of Wellington says just because galaxy clusters are big doesn’t mean they are easy to see – they are very old, very far away and very faint.

Melanie uses telescopes to look at different parts of the electromagnetic spectrum coming from galaxies and galaxy clusters. She is very interested in radio waves and X-rays, as well as visible light.

To see the visible light from galaxies, Melanie uses the largest optical telescope in Australia, the 3.9 m Anglo-Australian Telescope. This lets her find the galaxies, but what she really wants to know is what’s going on between the galaxies. Knowing this will let Melanie explore processes of galaxy cluster evolution. To do this, Melanie collects data using a radio telescope in Australia (the Australia Telescope Compact Array) and an X-ray observatory in orbit around Earth (the XMM-Newton, run by the European Space Agency). Fortunately, Melanie doesn’t have to travel to these locations to use the equipment – she controls them from her office computer in Wellington.

The images that Melanie has collected from galaxy clusters have revealed some surprises.

One galaxy cluster that Melanie has studied is called Abell 3667, which has about 500 galaxies. The giant galaxy that pulls the cluster together is in the middle of a cloud of very hot gas (10 million degrees Kelvin), which emits X-rays.

Radio waves revealed shock waves 6 million light years across spreading out through the hot gas between galaxies. This was rare evidence of two galaxy clusters colliding. Imagine the huge amounts of energy released when two of the largest structures in the universe collide

I need a bigger telescope!

Astronomers know of many galaxy clusters that have collided, but few of the collisions seem to have emitted the radio waves that Melanie measured. This could be because the radio waves are so faint that our instruments don’t pick them up – it took Melanie 108 hours to collect the data for only one image. Melanie would like a more powerful radio telescope so that she, and astronomers like her, can see these faint objects in more detail. The data Melanie provides is used to build models of the evolution of the universe, so more accurate data will mean more reliable models.

The Square Kilometre Array

Melanie’s wish for a more powerful radio telescope will soon be answered. If you think of the dish of a radio telescope as a bucket for collecting radio waves, what is needed is a bigger bucket. The Australian radio telescope Melanie uses at the moment has six 22 m wide dishes, with a combined collecting bucket of 2500 square metres. Seventeen countries have got together to build a group of up to 4,400 radio telescopes that will give a combined bucket of 1 square kilometre – that’s 1 million square metres. By using lots of small telescopes together, we can simulate a continent-sized radio telescope with 10 000 times more accuracy than the largest single radio telescope that can be built. The Square Kilometre Array will be built in either southern Africa, or Australia and New Zealand, but we won’t know where until 2011.