Forensic reconstruction of galaxy colour evolution and population characterisation

Mapping the evolution of galaxy colours, from blue star-forming to red passive systems, is fundamental to understand the processes involved in galaxy evolution. To this end, we reconstruct the colour evolution of low-redshift galaxies, combining stellar templates with star formation and metallicity histories of galaxies from the Galaxy And Mass Assembly survey and Shark semi-analytic model. We use these colour histories to robustly characterise the evolution of red and blue galaxy populations over cosmic time. Using a Gaussian Mixture Model to characterise the colour distribution at any given epoch and stellar mass, we find both observations and simulations strongly favour a model with only two populations (blue and red), with no evidence for a third "green" population. We map the evolution of mean, weight, and scatter of the blue and red populations as a function of both stellar mass and lookback time. Using our simulated galaxy catalogue as a testbed, we find that we can accurately recover galaxies colour histories up to a lookback time of $\sim6$ Gyr. We find that both populations show little change in the mean colour for low-mass galaxies, while the colours at the massive end become significantly redder with time. The stellar mass above which the galaxy population is predominantly red decreases by $0.3$ dex in the last $5$ Gyrs. We find a good agreement between observations and simulations, with the largest tension being that massive galaxies from Shark are too blue (a known issue with many galaxy evolution models).