HST/WFC3 Confirmation of the Inside-out Growth of Massive Galaxies at 0 < z < 2 and Identification of Their Star-forming Progenitors at z ~ 3

We study the structural evolution of massive galaxies by linking progenitors and descendants at a constant cumulative number density of n $_c$ = 1.4 { imes} 10$^{-4}$ Mpc$^{-3}$ to z ~{} 3. Structural parameters were measured by fitting Sersic profiles to high-resolution CANDELS HST WFC3 J $_{125}$ and H $_{160}$ imaging in the UKIDSS-UDS at 1 {lt} z {lt} 3 and ACS I $_{814}$ imaging in COSMOS at 0.25 {lt} z {lt} 1. At a given redshift, we selected the HST band that most closely samples a common rest-frame wavelength so as to minimize systematics from color gradients in galaxies. At fixed n $_c$, galaxies grow in stellar mass by a factor of ~{}3 from z ~{} 3 to z ~{} 0. The size evolution is complex: galaxies appear roughly constant in size from z ~{} 3 to z ~{} 2 and then grow rapidly to lower redshifts. The evolution in the surface mass density profiles indicates that most of the mass at r {lt} 2 kpc was in place by z ~{} 2, and that most of the new mass growth occurred at larger radii. This inside-out mass growth is therefore responsible for the larger sizes and higher Sersic indices of the descendants toward low redshift. At z {lt} 2, the effective radius evolves with the stellar mass as r$_e$ vpropM $^{2.0}$, consistent with scenarios that find dissipationless minor mergers to be a key driver of size evolution. The progenitors at z ~{} 3 were likely star-forming disks with r$_e$ ~{} 2 kpc, based on their low Sersic index of n ~{} 1, low median axis ratio of b/a ~{} 0.52, and typical location in the star-forming region of the U - V versus V - J diagram. By z ~{} 1.5, many of these star-forming disks disappeared, giving rise to compact quiescent galaxies. Toward lower redshifts, these galaxies continued to assemble mass at larger radii and became the local ellipticals that dominate the high-mass end of the mass function at the present epoch. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555.