The Growth of Massive Galaxies Since z=2
  We study the growth of massive galaxies from z=2 to the present using data
from the NEWFIRM Medium Band Survey. The sample is selected at a constant
number density of n=2x10^-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)=11.45-0.15z. In order to determine at what physical radii this
mass growth occurred we construct very deep stacked rest-frame R-band images 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 5-75 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 build-up from z=2 to z=0. 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.
