Spectral emission explains galactic evolution

Astronomers have measured the intensity of oxygen emission lines over the history of the universe to better understand how galaxies form.

Researchers from the University of California measured ionised oxygen levels from 12.5bn years ago to the present day and discovered ionised oxygen strength – intensity on emission spectra — increased as they looked further back in time. These results will help future astronomers better understand conditions inside star-forming galaxies.

Lead author of the paper and California University graduate student Ali Ahmad Khostovan, said: “Future telescopes, such as JWST and the Thirty Meter Telescope, will help shed more light on the internal properties of galaxies with their state-of-the-art spectrographs, which will have better spectral resolution to effectively study the ionising conditions in star-forming regions.

“In the case of JWST (to be launched in October 2018), we can peer even further into the past and observe oxygen emission lines from galaxies even farther away, that would fall into the infrared part of the spectrum, which is not accessible to ground-based telescopes. The advantage is that we can trace the ionising conditions of galaxies up to when galaxy formation began.”

Galaxies can be visualised as large areas of cold gas that produce gas, with some more animated than others. The data the scientists have collected will help explain how the cold gas has changed over time and how that is linked to star formation.

Khostovan said: “The fact that the universe is becoming less active in producing stars tells us that there will probably be no new generation of stars in the distant future. This means that many billions of years from now, when the least massive stars have died out, the universe would essentially become dark and devoid of stars. For astronomers, the fact that galaxies are becoming less active in star formation presents a major puzzle, as we want to understand what physical processes are involved to create such a change in activity.”

The data was taken from the High-Emission Line Survey (HiZELS). The authors concluded the strength of doubly ionised oxygen increased going back in time to more than 12bn years ago. But the strength of singly ionised oxygen varied – it increased back in time up to 11bn years ago before decreasing for the remaining 1.5bn years.

The study was carried out in conjunction with astronomers across the UK and US and was published in Monthly Notices of the Royal Astronomical Society.