2021ApJ...919..139W

A fundamental feature of galaxies is their structure, yet we are just now understanding the evolution of structural properties in quantitative ways. As such, we explore the quantitative nonparametric structural evolution of 16,778 galaxies up to z ∼ 3 in all five CANDELS fields, the largest collection of high-resolution images of distant galaxies to date. Our goal is to investigate how the structure, as opposed to size, surface brightness, or mass, changes with time. In particular, we investigate how the concentration and asymmetry of light evolve in the rest-frame optical. To interpret our galaxy structure measurements, we also run and analyze 250 simulation realizations from IllustrisTNG to determine the timescale of mergers for the CAS system. We measure that from z = 0-3, the median asymmetry merger timescale is 0.50_–0.18^+0.33 Gyr, and find that it does not vary with redshift. Using these data, we find that galaxies become progressively asymmetric at a given mass at higher redshifts, and we derive merger rates that scale as ∼(1+z)^1.87±0.04^ Gyr^–1, which agrees well with recent machine-learning and galaxy-pair approaches, removing previous inconsistencies. We also show that far-infrared selected galaxies that are invisible to the Hubble Space Telescope have a negligible effect on our measurements. We also find that galaxies are more concentrated at higher redshifts. We interpret this as a sign that their formation occurs from a smaller initial galaxy that later grows into a larger one through mergers, consistent with the size growth of galaxies from the inside out, suggesting that the centers are the oldest parts of most galaxies.