By studying objects at extreme distances, astronomers can learn more about the universe’s early development and how the very first galaxies were formed. Because it takes many billions of years for the light from the most distant galaxies to reach the Earth, we see them as they looked a very long time ago, close to the time of the Big Bang. It is in the light spectrum from the galaxy SXDF-NB1006-2 that the telescope ALMA (Atacama Large Millimeter/submillimeter Array) in Chile has now discovered an emission line from the element oxygen – at a greater distance and thus from a time closer to the Big Bang than has ever been found before. The galaxy is located at such a distance that the light intercepted has taken about 13.1 billion years to get there. This means that we see the galaxy as it looked about 700 million years after the Big Bang.
Immediately after the Big Bang, there were only the light elements hydrogen, helium and a small amount of lithium. All of the other, heavier, elements were formed later inside stars and spread throughout space through supernova explosions and stellar winds. With each new generation of stars born, the amount of heavy elements in the universe, such as carbon and oxygen, increases. By studying heavy elements in the most distant galaxies, we can learn more about the early stages of this enrichment and also get information about the properties of both gas and stars in these galaxies.
‘This discovery is important for two reasons,’ says Erik Zackrisson, Associate Professor of Astronomy at Uppsala University and one of the co-authors of the article. ‘The first has to do with the determination of distance, and the other with the galaxy’s role in the re-ionisation of the universe.’
If you can identify emission lines in the light spectrum from galaxies, you can also estimate how far away the galaxies are. The problem with the most distant objects is that they are so faint that even the largest telescopes usually fail to discover any lines at all.
‘The key is to know which elements and which lines you should look for,’ says Erik. ‘When the ALMA telescope was completed a few years ago, many people thought that it would help us discover galaxies at record distances, but that hasn’t happened yet. The competition to use ALMA is fierce and most researchers have invested their expensive observation time in what they believed was the safest bet – a line from the element carbon, which is usually the strongest in more nearby galaxies. Computer simulations of the first galaxies, however, predicted that a line from the element oxygen should be even stronger – and now we’ve proven that is true. I think this will open the door for lots of exciting discoveries of galaxies at record distances in the coming years.’
Measurements of the SXDF-NB1006-2 galaxy using the ALMA telescope can also provide important insight into the mystery of how the universe re-ionised. Immediately after the Big Bang, the universe was very hot. As it expanded, it gradually cooled, and after about 380,000 years, protons and electrons could finally be joined to form neutral hydrogen. A bit later, at some point during the first billion years after the Big Bang, space was filled with energetic photons, light particles, which broke up the hydrogen bonds and caused the re-ionisation of the universe.
Exactly how this happened, and where the ionising photons came from, is still unknown. Many people believe that it was young, hot stars in the first galaxies that created the photons that re-ionised the universe. But for this to be possible, the high-energy radiation would have had to escape the actual galaxies and reach the large amounts of gas between the galaxies in the intergalactic medium. Neutral hydrogen and stardust inside the galaxies can prevent the radiation from escaping, and examinations of galaxies in the more nearby parts of space have long showed that leakage of ionising radiation seems to be extremely rare. But perhaps the properties of the first galaxies are different?
The observations of galaxy SXDF-NB1006-2 show a lack of both stardust and neutral gas, which means that large amounts of ionised photons from hot stars should be able to leak out.
‘If lots of galaxies in the early universe have properties similar to SXDF-NB1006-2, we may be close to solving the mystery of the re-ionisation of the universe,’ says Erik Zackrisson.