OSB archive
OSB archive

A 'compound eye' on the sky

Pete Wilton

At ESO's Very Large Telescope (VLT) in Chile they are about to fit a new instrument that can record the light from 24 galaxies simultaneously.

KMOS has 24 robotic arms tipped with gold-plated mirrors that can be trained on a different galaxy – each arm has almost 200 facets making them rather like an insect's compound eye. Light from these mirrors is channelled into 3 spectrographs and 'multiplexed' – combined into a single signal.

The 3 spectrographs were designed, manufactured, and assembled at Oxford University before being shipped out to Chile via STFC's UK Astronomy Technology Centre in Edinburgh.

Working at infrared wavelengths, KMOS will probe a crucial time in the evolution of galaxies: around 10 billion years ago when star formation was at its height and the black holes believed to nestle in the centres of most galaxies were also highly active.

'Not only will KMOS accelerate the study of high redshift galaxies through the multiplex advantage, it will also provide a much more detailed view, allowing us to study gas flows and star forming regions in each individual galaxy,' Roger Davies, who led work at Oxford on the KMOS spectrographs, explains. 'We expect that these will reveal the connection between the evolution of the stars in galaxies and central black hole.'

The Oxford team are particularly interested in looking at galaxies in rich clusters: swarms of galaxies that occupy a compact volume and so live in a dense environment at a common distance.

In the nearby Universe these cosmic laboratories host large populations of structureless galaxies that appear to have completed almost all their star formation. 'KMOS will help us to identify the physical processes that give rise to this particular population of galaxies in clusters,' Roger tells me.

Over the last decade the spectrometers for KMOS were designed and constructed in Oxford University's Department of Physics. An experienced team of Ian Lewis, Matthias Tecza and Niranjan Thatte, as well as Davies, have established a strong track record for Oxford in instruments of this kind having built instruments for the the Mt Palomar 5m and the Japanese Subaru 8m telescope in recent years.

Building KMOS was a huge technical challenge: 'The whole interior of the instrument is cryogenic – cooled to 100 Kelvin [-173 Celsius] – so the spectrographs have to work at these extreme temperatures,' Ian Lewis tells me. The Oxford team worked closely with colleagues at the Rutherford Appleton Laboratory on the optical design of KMOS, whilst the Thin Film Facility gave it its golden glow – gold-plating all the mirrors for the device.

KMOS can detect emissions from gas at very early times in the history of the Universe that is normally invisible in images of the sky. The light from gas at high redshift can be concentrated in emission at a single wavelength, when an image is recorded over a broad range of wavelengths this light can be swamped and not detectable above the background glow. Because KMOS spreads the light out in wavelength over an area of sky, it can potentially detect the sharp emission lines from the most distant gas known, 'This could be one of the most exciting results from KMOS,' Roger adds.

The instrument is a collaboration of six institutions in Germany and the UK, including STFC's UK Astronomy Technology Centre, Durham University, Oxford University and RAL Space at STFC's Rutherford Appleton Laboratory.