Free Astronomy Magazine January-February 2016

SPACE CHRONICLES axies the team discovered that the accumulated light emitted by these galaxies could have played a major role in one of the most mysterious periods of the Universe’s early his- tory — the epoch of reionisation. Reionisation started when the thick fog of hydrogen gas that cloaked the early Universe began to clear. Ultraviolet light was now able to travel over larger distances without being blocked and the Universe became transparent to ultraviolet light. By observing the ultraviolet light from the galaxies found in this study the astronomers were able to calcu- late whether these were in fact some of the galaxies involv- ed in the process. The team determined, for the first time with some confidence, that the smallest and most abundant of the gal- axies in the study could be the major actors in keeping the Universe transparent. By doing so, they have establish- ed that the epoch of reionisation — which ends at the point when the Universe is fully transparent — came to a close about 700 mil- lion years after the Big Bang (this corresponds to a redshift of about z = 7.5). Lead author Atek explained, “If we took into account only the contributions from bright and massive gal- axies, we found that these were insufficient to reionise the Uni- verse. We also needed to add in the contribu- tion of a more abun- dant population of faint dwarf galaxies.” Hubble spies Big Bang frontiers by NASA A n international team of as- tronomers, led by Hakim Atek of the Ecole Polytech- nique Fédérale de Lausanne, Swit- zerland, has discovered over 250 tiny galaxies that existed only 600-900 million years after the Big Bang — one of the largest samples of dwarf galaxies yet to be discovered at these epochs. The light from these galaxies took over 12 billion years to reach the telescope, allowing the astrono- mers to look back in time when the uni- verse was still very young. The calculated redshift for these ob- jects is between z = 6 and z = 8. Although impressive, the number of galaxies found at this early epoch is not the team’s only remarkable break- through, as Johan Ri- chard from the Obser- vatoire de Lyon, France, points out, “The faint- est galaxies detected in these Hubble observa- tions are fainter than any other yet uncover- ed in the deepest Hub- ble observations.” By looking at the light coming from the gal- T his image from the NASA/ESA Hubble Space Telescope shows the galaxy cluster MACS J0416.1–2403. This is one of six being studied by the Hubble Frontier Fields programme, which together have produced the deepest images of gravitational lensing ever made. [NASA, ESA and the HST Frontier Fields team (STScI)]