A spectrum paints a thousand pictures: ancient galaxies viewed in 3 dimensions
Dr Ian Parry and Dr Andrew Bunker.
University of Cambridge.
Astronomers can now watch the behaviour of ancient galaxies in greater detail than ever before using an instrument built by researchers in Cambridge. The instrument splits images from large telescopes into tiny elements that are analysed separately to reveal the motion and even chemical composition of distant stars.
Watching distant galaxies is the same as watching an event a long time ago, as their light takes time to reach us. Astronomers need very large telescopes to collect the weak light from distant galaxies, which spreads out as it travels like ripples across a pond. But to gain as much information as possible from the images collected by these telescopes, Dr Ian Parry of the University of Cambridge and his colleagues have built CIRPASS - the Cambridge Infrared Panoramic Survey Spectrograph. When hooked up to one of the largest telescopes in the world, this instrument can reveal the behaviour of galaxies some 8 billion years old - when the Universe itself was a mere 6 billion years old.
The spectrum of light from a distant object holds a wealth of information about how it is moving and what it is made of. Astronomers use a spectrograph to collect this information, spreading out the light into its spectrum. Normally spectrographs use slits to measure the spectra of lines in telescope images. But CIRPASS measures a spectrum for individual points in an image to provide a much more detailed picture. 'CIRPASS gets the whole picture, not just a part of it,' says Parry.
The light from a single pixel is funneled by a lens into an optical fibre - a flexible pipe for carrying light. The light coming out of each fibre is then spread into its colours by the spectrograph - which is kept in a freezer at -40ºC to improve its performance - to produce a spectrum for each pixel.
Having a spectrum for each pixel adds a third dimension of information to the two-dimensional telescope image, allowing astronomers to deduce how the stars in ancient galaxies moved, how quickly they formed and what they were made of.
Stars were born more often in the early Universe than they are now. CIRPASS can peer back into the past to study this process in greater detail than ever before. 'CIRPASS can measure the birthrate and see where inside the galaxies the stars were being born,' Parry explains. And by measuring the motion of stars in ancient galaxies, the instrument can also reveal how the galaxies themselves were formed.