Free Astronomy Magazine May-June 2016

SPACE CHRONICLES T his illustration shows a timeline of the Universe, stretching from the present day (left) back to the Big Bang, 13.8 billion years ago (right). The newly found galaxy GN-z11 is the most distant galaxy discovered so far, at a redshift of 11.1, which corresponds to 400 million years after the Big Bang. The previous record holder’s position is also identi- fied. Its remote position puts GN-z11 at the beginning of the reionisation era. In this period starlight from the first galaxies started to heat and lift the fog of cold hydrogen gas filling the Universe. The previous record-holding galaxy was seen in the middle of this epoch, about 150 million years later. [NASA, ESA, and A. Feild (STScI)] nadian Space Agency (CSA). It is sche- duled for launch in 2018. “We’ve taken a major step back in time, beyond what we’d ever ex- pected to be able to do with Hubble. We managed to look back in time to measure the distance to a galaxy when the Universe was only three percent of its current age,” says Pas- cal Oesch of Yale University and lead author of the study. To determine large distances, like the one to GN-z11, astronomers measure the redshift of the observed object. This phenomenon is a result of the expansion of the Universe; every di- stant object in the Universe appears to be receding from us and as a re- sult its light is stretched to longer, redder wavelengths. Before astronomers determined the distance to GN-z11, the most distant measured galaxy, EGSY8p7, had a redshift of 8.68. Now, the team has confirmed GN-z11’s distance to be at a redshift of 11.1, which corresponds to 400 million years after the Big Bang. “The previous record-holder was seen in the middle of the epoch when starlight from primordial gal- axies was beginning to heat and lift a fog of cold, hydrogen gas,” ex- plains co-author Rychard Bouwens from the University of Leiden, the Netherlands. “This transitional pe- riod is known as the reionisation era. GN-z11 is observed 150 million years earlier, near the very beginning of this transition in the evolution of the Universe.” The combination of ob- servations taken by Hubble and Spit- zer revealed that the infant galaxy is 25 times smaller than the Milky Way and has just one percent of our gal- axy’s mass in stars. However, the number of stars in the newborn GN- z11 is growing fast. The galaxy is forming stars at a rate about 20 times greater than the Milky Way does today (about 24 solar masses of gas and dust per year into new stars). This high star formation rate makes the remote galaxy bright enough for Hubble to see and to perform de- tailed observations. However, the discovery also raises many new ques- tions as the existence of such a bright and large galaxy is not predicted by theory. “It’s amazing that a galaxy so massive existed only 200 million to 300 million years after the very first stars started to form. It takes really fast growth, producing stars at a huge rate, to have formed a galaxy that is a billion solar masses so soon,” explains Garth Illingworth of the University of California, Santa Cruz. Marijn Franx, a member of the team from the University of Leiden high- lights: “The discovery of GN-z11 was a great surprise to us, as our earlier work had suggested that such bright galaxies should not exist so early in the Universe.” His colleague Ivo Labbe adds: “The discovery of GN- z11 showed us that our knowledge about the early Universe is still very restricted. How GN-z11 was created remains somewhat of a mystery for now. Probably we are seeing the first generations of stars forming around black holes?” These findings provide a tantalising preview of the observations that the James Webb Space Telescope will per- form. “This new discovery shows that JWST will surely find many such young galaxies reaching back to when the first galaxies were form- ing,” concludes Illingworth. n