“The discovery that young galaxies are so unexpectedly bright–if you look for this distinctive green light–will dramatically change and improve the way that we study Galaxy formation throughout the history of the Universe.” -Matthew Malkan
Galaxies come in many different colors today: white, blue or red, mostly, depending on the populations of stars inside. But in a very rare set of circumstances, there can be green emissions as well, due mostly to the emission lines of doubly-ionized oxygen. The problem is we only see these in extraordinarily hot regions of the Universe, where ultra-hot stars or extremely unusual ultraviolet processes are found.
The strong green emission line (highest point) as shown in a sample of over 1,000 galaxies, spectrally stacked from the Subaru Deep Field. The other point “above” the curves is from hydrogen; the strong green oxygen line indicates incredibly intense radiation. Image credit: Malkan and Cohen (2017).
Yet by looking at the farthest star-forming galaxies available, two scientists broke the light up into components and found something shocking: this green emission feature was found to be incredibly strong in every one of thousands of galaxies that fit the criteria. It leads to a huge mystery: how do these ultra-hot stars form in the first place? Is it metallicity? Higher stellar masses? Or a top-heavy initial mass function?
The reionization and star-formation history of our Universe. The study hints that green, oxygen-rich galaxies may have been responsible for reionization. Image credit: NASA / S.G. Djorgovski & Digital Media Center / Caltech.