While a host of astronomical objects could in principle have contributed to this process, star-forming galaxies are usually seen as the top candidates, since they are already known to exist in great numbers at the relevant epoch. However, this scenario hinges on the assumption that ionizing radiation can escape from these objects and into the intergalactic medium. Even if ionizing photons can make it out of primordial galaxies, these photons are absorbed before reaching us. So how does one put this idea to the test?

In a study published in the November 2013 issue of the Astrophysical Journal, Erik Zackrisson, Hannes Jensen (both from Stockholm University) and Akio Inoue (Osaka Sangyo University) argue that even if the ionizing photons themselves cannot be detected, the escape of ionizing radiation gives rise to tell-tale signatures in the spectra of galaxies throughout the reionization epoch. These spectral features can be studied with the upcoming James Webb Space Telescope (JWST), scheduled for launch in 2018. This result opens up new potential avenues for reionization studies with JWST and may be pivotal for the role of JWST in cracking the reionization puzzle.

JWST – a joint project of NASA, the European Space Agency and the Canadian Space Agency – will be the largest telescope ever launched into space and will be placed in a location about four times further from Earth than the Moon. Sweden has been involved in the development of MIRI, one of instruments that will help JWST study the assembly of galaxies in the early Universe.

Read the full research paper, or the pre-print version.

Astronomers at the Oskar Klein Centre, Stockholm University, are organizing an international workshop devoted to the study of cosmic reionization in August 2014.