Half-mass Radii of Quiescent and Star-forming Galaxies Evolve Slowly from 0&xfffd;?&xfffd;z&xfffd;?&xfffd;2.5: Implications for Galaxy Assembly Histories** This work is based on observations taken by the CANDELS Multi-Cycle Treasury Program and the 3D-HST Treasury Program with the NASA/ESA HST, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

We use high-resolution, multiband imaging of ?16,500 galaxies in the CANDELS fields at 0 & xfffd;?& xfffd;z & xfffd;?& xfffd;2.5 to study the evolution of color gradients and half-mass radii over cosmic time. We find that galaxy color gradients at fixed mass evolve rapidly between z & xfffd;?& xfffd;2.5 and z & xfffd;?& xfffd;1, but remain roughly constant below z & xfffd;?& xfffd;1. This result implies that the sizes of both star-forming and quiescent galaxies increase much more slowly than previous studies found using half-light radii. The half-mass radius evolution of quiescent galaxies is fully consistent with a model that uses observed minor merger rates to predict the increase in sizes due to the accretion of small galaxies. Progenitor bias may still contribute to the growth of quiescent galaxies, particularly if we assume a slower timescale for the minor merger growth model. The slower half-mass radius evolution of star-forming galaxies is in tension with cosmological simulations and semianalytic galaxy models. Further detailed, consistent comparisons with simulations are required to place these results in context.