Galaxies produce stars from cold gas, but some galaxies are more productive than others and their productivity changes across cosmic time.

Overall, galaxies seem to have been the most productive 2-3 billion years after the Big Bang, with a consistent decline ever since. In other words, the Universe seems to be in quite a production crisis as galaxies are becoming less active in creating new stars. However, understanding why that is the case is still an open question.

“One way to make breakthroughs is to time travel to different epochs of the Universe, observe galaxies and their properties, and then try to figure out what is changing and why. So that’s exactly what we did”, comments astrophysicist Dr David Sobral from Lancaster University who is a co-author of the research in the Monthly Notices of the Astronomical Society.

“By using four very specific filters, we were able to use some of the best telescopes in the World to time-travel to four key epochs of the Universe and study the properties of the hot/ionised gas, including Oxygen”.

“These lines are produced when very high energy light/photons interact with the surrounding medium resulting in regions where atoms are broken up (ionized). As atoms recombine, they can glow bright in what our eyes would see as green light,” said lead author Ali Ahmad Khostovan, a graduate student at the Department of Physics and Astronomy, UC Riverside.

In the study, astronomers have used a sample of emission-line selected galaxies from the High-z Emission-Line Survey (HiZELS) to trace the evolution in the strengths of emission lines associated with ionised Oxygen.

By studying how distant Oxygen atoms recombine in very distant galaxies, astronomers were able to investigate the sources and nature of distant galaxies, from the present-day and up to 12.5 billion years ago, for the first time.

Dr Sobral said: “The results show that the amount of ionising photons produced by galaxies in the early Universe is much higher than those we see in today’s galaxies, resulting in distant galaxies glowing more and more in Oxygen lines as we approach the very early days of galaxy formation.”

The results of this study help to set the framework for future surveys using the next-generation telescopes, such as the upcoming James Webb Space Telescope (JWST), that will allow a study of conditions inside star-forming galaxies even further into the past to the era of the first galaxies.

The paper was co-authored by David Sobral [Lancaster University], Bahram Mobasher [UC Riverside], Ian Smail [Durham University], Behnam Darvish [Caltech], Hooshang Nayyeri [UC Irvine], Shoubaneh Hemmati [IPAC/Caltech], and John Stott [Oxford University].