THE GROWTH OF MASSIVE GALAXIES SINCEz= 2

We study the growth of massive galaxies from z = 2 to the present using data from the NOAO/Yale NEWFIRM Medium Band Survey. The sample is selected at a constant number density of n = 2 × 10 −4 Mpc −3 , so that galaxies at different epochs can be compared in a meaningful way. We show that the stellar mass of galaxies at this number density has increased by a factor of ≈2 since z = 2, following the relation log M n ( z ) = 11.45 − 0.15 z . In order to determine at what physical radii this mass growth occurred, we construct very deep stacked rest-frame R -band images of galaxies with masses near M n ( z ), at redshifts 〈 z 〉 = 0.6, 1.1, 1.6, and 2.0. These image stacks of typically 70–80 galaxies enable us to characterize the stellar distribution to surface brightness limits of ∼28.5 mag arcsec −2 . We find that massive galaxies gradually built up their outer regions over the past 10 Gyr. The mass within a radius of r = 5 kpc is nearly constant with redshift, whereas the mass at 5 kpc < r < 75 kpc has increased by a factor of ∼4 since z = 2. Parameterizing the surface brightness profiles, we find that the effective radius and Sersic n parameter evolve as r e ∝ (1 + z ) −1.3 and n ∝ (1 + z ) −1.0 , respectively. The data demonstrate that massive galaxies have grown mostly inside-out, assembling their extended stellar halos around compact, dense cores with possibly exponential radial density distributions. Comparing the observed mass evolution to the average star formation rates of the galaxies we find that the growth is likely dominated by mergers, as in situ star formation can only account for ∼20% of the mass buildup from z = 2 to z = 0. A direct consequence of these results is that massive galaxies do not evolve in a self-similar way: their structural profiles change as a function of redshift, complicating analyses which (often implicitly) assume self-similarity. The main uncertainties in this study are possible redshift-dependent systematic errors in the total stellar masses and the conversion from light-weighted to mass-weighted radial profiles.