A region of Abell 1758, a massive galaxy cluster photographed by the RELICS project As the fireball of the Big Bang expanded and cooled, it went from white-hot to cherry-red before finally fading into invisibility. The Universe was plunged into blackness and the resulting cosmic dark age stretched on interminably. Over time, the Universe doubled in size, doubled once more, over and over again, then, one day, something extraordinary happened. The dark age came to an end. Across the entire length and breadth of the Universe, stars began switch on like lights on a Christmas tree. The first stars either came together under gravity to create the first galaxies, or were actually born in the clouds of gas and dust that made up the first galaxies. And the hunt to find these first galaxies is hotting up. One project – the Re-ionization Lensing Cluster Survey (RELICS) – has found around 300 galaxies that existed in the first billion years of the Universe’s history. One galaxy in particular is so old that the Universe it occupied was a mere 3 per cent of its present age of 13.82 billion years. Such objects appear in astronomers’ telescopes like persistent after-images, their light having travelled across space for billions upon billions of years before reaching us.
BACK IN TIME
More than 40 astronomers in many countries have been involved in the RELICS project, contributing hundreds of hours of observing time on the Hubble Space Telescope and the Spitzer Space Telescope. However, the principal observing instrument is the Universe itself. The gravitational fields of the massive clusters of galaxies that pepper the Universe act like giant lenses that focus and magnify the light of more distant galaxies that are often far too faint to see by any other means. “We take advantage of nature’s own telescope,” says Dan Coe, principal investigator of RELICS at the Space Telescope Science Institute in Baltimore. To find the useful lensing clusters that would help him spot the oldest galaxies, Coe searched through Hubble’s archive of images and a recent catalogue of around 1,000 galaxy clusters observed by the European Space Agency’s (ESA’s) Planck satellite. Planck’s principal purpose was to image the cosmic background radiation – the ‘afterglow’ of the Big Bang fireball itself – but the ‘far-infrared’ light it picked up also comes from warm dust in galaxy clusters. “We ended up with 41 massive galaxy clusters,” says Coe. “We selected them for their extreme mass, which makes them enormously powerful gravitational lenses.” In the immediate neighbourhood of each cluster are literally thousands of ghostly images of distant galaxies that by chance have been ‘gravitationally lensed’ by the cluster. But most are of galaxies that are not at great distances,and so not in the very early Universe. “The trick to finding truly ancient objects is to look for lensed galaxies that appear in infrared images from Hubble and Spitzer images, but not in Hubble images taken at visible wavelengths,” says Coe. But to understand why this reveals ultra-distant, ultraearly galaxies, we need to look at the concept of ‘redshift’.
THE MOST DISTANT GALAXY IN RELICS EXISTED OVER 13 BILLION YEARS AGO
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