Most galaxies are either actively forming stars or quenched, but there is a small number of galaxies in transition from one population to the other. These galaxies are "quenching" if they are in the process of becoming quenched or "rejuvenating" if they are returning to the star-forming main sequence after a period of being quenched. Quenching occurs when a galaxy’s limited cold gas supply is heated or removed, halting star formation, while rejuvenation refers to any process that reintroduces cold gas to quenched galaxies, reigniting star formation. Rejuvenating galaxies, which are significantly rarer and less well-studied than quenching galaxies, can offer valuable insights into galaxy evolution processes. This thesis investigates the properties of transitional galaxies, with a focus on their gas content, to explore the mechanisms driving quenching and rejuvenation. We employ a recent classification method using GALEX NUV and Sloan Digital Sky Survey H-alpha measurements to identify transitional galaxies and analyze the derived gas properties of those in the xGASS and xCOLD GASS surveys. We find that rejuvenating and quenching galaxies have intermediate gas fractions compared to actively star-forming and quenched galaxies, and that rejuvenating and quenching galaxies have similar depletion times to star-forming and quenched galaxies, respectively. We also find that the rejuvenating population, particularly at lower stellar mass, is efficient at converting its atomic gas supply to molecular hydrogen, which could be attributed to their high gas-phase metallicities at low stellar mass.

Master of Science (MSc)

Thesis