Disputing the green valley theory of galaxy evolution

Fully understanding the process of galaxy evolution has long been a conundrum in astronomical research, given the limited amount of precise observational data and the complexity of processing the many factors that lead to the development of a galaxy. These include a galaxy’s many shapes or morphologies, the presence of dark matter, the interactions between galaxies, and the distance at which galaxies can be observed. A crossroad in elucidating galaxy evolution—where star-forming “blue” galaxies transition to quiescent “red” galaxies—exists between the green valley and red-herring hypotheses. The former theory postulates that the green valley is a significant step in galaxy evolution, specifically, a transitory phase between “blue” and “red” galaxies. The latter asserts that the green valley is not an actual phase but a visual consequence of that unseen transition. This study aimed to evaluate the validity of these two theories. The Set of Identifications, Measurements, and Bibliography for Astronomical Data (SIMBAD) and the NASA Extragalactic Database (NED) were used to mine galaxies (N=293) that were then sorted into three bins: normal spirals, barred spirals, and ellipticals. We analyzed the ultraviolet-red color index means of the three bins and, after examining correlations between the means and galaxy subtypes, found a split between early- and late-type galaxy evolution. As a result, this study supports the red-herring hypothesis, with evidence that the green valley is only a byproduct of transitioning galaxies. Future research may examine quenching timescales and the effects of specific morphological features on galaxy evolution processes.