Empirical Model for the Size Growth of Galaxies

Since the timescales for galaxy evolution are too long to follow observationally for individual galaxies, empirical models provide a powerful tool to constrain the physical processes that drive galaxy growth. Here, we present an empirical model that comprehensively captures the principal drivers of galaxy evolution through star formation and stellar mass growth on spatially resolved scales, allowing us to describe morphological evolution with cosmic time. In the model, star formation and stellar mass evolve based on the star-forming main sequence, which relates the star formation rate (SFR) and stellar mass of a galaxy, and fluctuations relative to this main sequence relation to account for natural variations in growth throughout a galaxy’s lifetime. Star formation is suppressed probabilistically to produce the quiescent galaxy population. The morphological evolution is captured by assuming that the star formation is distributed in an exponential disk. The exponential disk profile has a free parameter for the scale length of the exponential shape, R_s, whose temporal and evolutionary dependencies we investigate. Analytical results from the model about the dependencies of R_s make novel contributions to understanding galaxy morphological evolution. With the model, we numerically construct a simulated sample of galaxies whose star formation and stellar mass evolution we compare to observational measurements of galaxies sizes. From this comparison, we learn how individual galaxies grow while they are star-forming, quenching, and in their quiescent phase.