Free Astronomy Magazine September-October 2015

SPACE CHRONICLES no ionised helium was detected; De Breuck et al., 2000, where ionised he- lium was detected, but alongside car- bon and oxygen, as well as clear signatures of an active galactic nu- cleus; and Cassata et al., 2013, where ionised helium was detected, but of a very low equivalent width, or weak intensity, and alongside carbon and oxygen. A team led by David Sobral, from the Institute of Astrophysics and Space Sciences, the Faculty of Sciences of the University of Lisbon in Portugal, and Leiden Observatory in the Ne- therlands, has now used ESO’s Very Large Telescope (VLT) to peer back into the ancient Universe, to a period known as reionisation, approximately 800 million years after the Big Bang. Instead of conducting a narrow and deep study of a small area of the sky, they broadened their scope to pro- duce the widest survey of very distant galaxies ever attempted. Their expan- sive study was made using the VLT with help from the W. M. Keck Obser- vatory and the Subaru Telescope as well as the NASA/ESA Hubble Space Telescope. The team discovered — and confirmed—a number of surpris- ingly bright very young galaxies. One of these, labelled CR7 (abbrevia- tion of COSMOS Redshift 7), was an exceptionally rare object, by far the brightest galaxy ever observed at this stage in the Universe. With the discovery of CR7 and other bright galaxies, the study was already a suc- cess, but further inspection provided additional exciting news. The X-shooter and SINFONI instru- ments on the VLT found strong ion- ised helium emission in CR7 but — crucially and surprisingly — no sign of any heavier elements in a bright pocket in the galaxy. This meant the team had discovered the first good evidence for clusters of Population III stars that had ionised gas within a galaxy in the early Universe. “The discovery challenged our expec- tations from the start,” said David So- bral, “as we didn’t expect to find such a bright galaxy. Then, by unveiling the nature of CR7 piece by piece, we un- derstood that not only had we found by far the most luminous distant gal- axy, but also started to realise that it had every single characteristic expec- ted of Population III stars. Those stars were the ones that formed the first heavy atoms that ultimately allowed us to be here. It doesn’t really get any more exciting than this.” Within CR7, bluer and somewhat red- der clusters of stars were found, indi- cating that the formation of Popu- lation III stars had occurred in waves — as had been predicted. What the team directly observed was the last wave of Population III stars, suggesting that such stars should be easier to find than previously thought: they reside amongst regular stars, in brighter galaxies, not just in the ear- liest, smallest, and dimmest galaxies, which are so faint as to be extremely difficult to study. Jorryt Matthee, second author of the study, concluded: “I have always won- dered where we come from. Even as a child I wanted to know where the elements come from: the calcium in my bones, the carbon in my muscles, the iron in my blood. I found out that these were first formed at the very be- ginning of the Universe, by the first generation of stars. With this discov- ery, remarkably, we are starting to ac- tually see such objects for the first time.” Further observations with the VLT, ALMA, and the NASA/ESA Hub- ble Space Telescope are planned to confirm beyond doubt that what has been observed are Population III stars, and to search for and identify further examples. T his artist’s impression shows CR7, a very distant galaxy dis- covered using ESO’s Very Large Telescope. It is by far the brightest galaxy yet found in the early Uni- verse and there is strong evidence that examples of the first genera- tion of stars lurk within it. These massive, brilliant, and previously purely theoretical objects were the creators of the first heavy elements in history — the elements necessary to forge the stars around us today, the planets that orbit them, and life as we know it. This newly found galaxy is three times brighter than the brightest distant galaxy known up to now. [ESO/M. Kornmesser] n