With gravitational lensing, researchers could locate population III stars in far galaxies.
The Big Bang explains the presence of the two lightest elements in the universe, hydrogen and helium, but the origin of many of the heavier elements that exist today remains unsolved.
A promising theory to explain their creation is the formation of population III stars. Population III stars would have formed from the hydrogen and helium available after the Big Bang and then created heavier elements in their cores. When these stars died, they would have exploded as supernovae, flinging the heavier elements they had created across the universe.
Finding population III stars would thus provide a crucial missing link in our understanding of the formation of the universe.
Too far away
Population III stars are notoriously difficult to find, despite a growing number of searches for them. “This class of stars has not yet been observed. Until now, when we think we have discovered two galaxies which may consist of Population III stars,” says Claes-Erik Rydberg from Stockholm University and lead author of a recent study searching for these stars.
Since population III stars would have formed very early on in the universe, they are by now too far away to be observed. So rather than searching for individual stars, Rydberg and his colleagues searched for entire galaxies made up of population III stars. Their approach was to first model what these galaxies would look like using computer simulations and then compare the predictions from their models to real observations taken by the Hubble Space telescope.
The observations the team used came from photographs of 25 clusters of galaxies. But it isn’t these galaxies that Rydberg is interested in. Instead, these galaxies are used as ‘gravitational lenses’ to reveal what lies behind them. The technique relies on a weird fact from general relativity: heavy things warp the space around them. As the 25 galaxy clusters are so massive, they bend the light from everything behind them and throw it forwards. The result is that these galaxy clusters reveal light from objects that are even further away from us, that would have otherwise been too faint for us to see.
Rydberg sifted through more than 50,000 of these objects. For each one, he compared the population III model to the real data to see if there was a match. “I started building the program a little more than 2 years ago,” says Rydberg. “Since then we have been searching and analysing possible objects.” After all the analysis, the team found two candidates that were best explained by the population III galaxy model rather than a more mundane galaxy.
Although these are the best candidates found to date, Rydberg cautions against claiming any discoveries too soon. There are no telescopes currently powerful enough to confirm whether the two galaxies really do contain population III stars. However, there is a critical test that can be done on Earth.
Population III stars are expected to emit a very specific type of light called the Lyman-alpha spectral line. If this light were observed coming from the two candidate galaxies using ground-based telescopes, it would be a strong hint that they really do contain population III stars. For Rydberg, this would be a huge result and could lead to future telescopes, such as NASA’s upcoming James Webb Space Telescope, being sent to investigate these two galaxies further for confirmation of the elusive population III stars.
Rydberg, C., Zackrisson, E., Zitrin, A., Guaita, L., Melinder, J., Asadi, S., Gonzalez, J., Östlin, G., & Ström, T. (2015). A SEARCH FOR POPULATION III GALAXIES IN CLASH. I. SINGLY IMAGED CANDIDATES AT HIGH REDSHIFT The Astrophysical Journal, 804 (1) DOI: 10.1088/0004-637X/804/1/13