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Clashing galaxy clusters and extreme energies

A galaxy is already incredibly large, but it can get even bigger. Astronomer Reinout van Weeren investigates clusters of galaxies, one of the largest structures in the universe. For his research into the origins and evolution of these clusters, he obtained an ERC Starting Grant of 1.5 million euros. ‘Our archive covers more than 30 Petabytes!’

Clusters of galaxies

Clusters are groups of thousands of galaxies and they therefore take up quite some space. ‘They are the largest structures in the universe that are still bound by gravity,' explains Van Weeren. He tries to understand how these structures arise and evolve. Although they are enormous, clusters move through the universe and can therefore collide with each other. As a result of these collisions, groups of galaxies join together, creating ever larger clusters. It’s this phenomenon Van Weeren is particularly interested in. There are only a few dozen collisions currently known. ‘With the radio-telescope we now want to map more or less all colliding clusters in the universe. We hope to discover several hundred of them.’

Extreme energies

During collisions, the electrons present are accelerated so much that they acquire a lot of energy. ‘You can compare it to the particle accelerators here on earth,' Van Weeren says. ‘After acceleration, these extremely energetic particles move almost at the speed of light. The higher the speed, the higher the energy.’ The energetic particles emit a specific radio radiation. ‘We can therefore use the radio telescope to detect particles that have once been accelerated. For example by colliding clusters, but also by black holes.’

Radio jets

Van Weeren suspects that black holes influence the evolution of galaxy clusters. This relates to the extremely energetic particles. An active black hole can create a radio jet, which blasts energetic particles into the galaxy like a kind of jet stream. This means that radio jets are also clearly visible with the radio telescope. Earlier research showed that these jets interact with the thin gas of the clusters and thus influence their structure. ‘What we do not yet understand, however, is how the jets transfer their energy to this thin gas.’

Radio jet at the constellation Herculas A. Visible light image obtained by the Earth-orbiting Hubble Space Telescope superposed with a radio image taken by the Very Large Array of radio telescopes in New Mexico, USA. NASA/ESA/Humble Heritage Team

Low frequency

For the research, Van Weeren and his colleagues make very detailed radio images with the LOFAR telescope in Drenthe, which observes at a very low frequency. This has never been done for galaxy clusters at this scale before. Van Weeren: ‘Previously, astronomers worked at much higher frequencies. But at lower frequencies the clusters are very bright and you can see the energetic particles much better. However, there is one big disadvantage: 'At an altitude of a few hundred kilometers, the earth is surrounded by a layer of ionised gas: the ionosphere. This layer disturbs low frequency radio radiation coming in from space. It’s a kind of noise that blurs our images. As if you were lying at the bottom of the pool, trying to look at the world outside.’ It is very complicated to correct this noise and make the images sharp again. It is only in the last few years that the computers have become powerful enough and the right techniques available.

Big data

Research into large structures results in large datasets that are just as large. ‘Our archive covers more than 30 Petabytes', Van Weeren says. AS a comparison: one Petabyte of MP3-ecoded songs,  would require 2000 years to play. Such large datasets are very challenging. For example, how do you download all that data from the supercomputer in Amsterdam? 'Data alone is of no use to us, we have to process it with the computer to finally create a picture out of it'. Downloading one observation takes so long that in the meantime, more new observations arrive than researchers can process. That is why they decided to carry out the processes at the supercomputer centre themselves. To this end, the researchers set up new software and systems. ‘We are now working in a sort of cloud environment. By no longer downloading every file, we are working more efficiently. This kind of knowledge is also very useful for other disciplines that work with big data.’

How big!?

1 Petabyte = 1000 Terabytes = 1,000,000,000 Megabytes

Clusters can grow up to 3 million light years in size. 1 lightyear is 9,460,000,000,000,000 km, which is equal to 236.07 million laps around the earth.

Cover photo: The Toothbrush Cluster'. In pink: the radio images from the LOFAR telescope (of which the shape resembles a toothbrush). Blue: X-rays observed by the Chandra-satellite. Visible light data from the Subaru-telescope show the galaxies and stars. Röntgen: NASA/CXC/SAO/R. van Weeren et al.; Radio: LOFAR/ASTRON; optisch: NAOJ/Subaru

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