Galaxy Stellar Mass Functions from ZFOURGE/CANDELS: An Excess of
  Low-Mass Galaxies Since z=2 and the Rapid Buildup of Quiescent Galaxies
  Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we
obtain the deepest measurements to date of the galaxy stellar mass function at
0.5 < z < 2.5. ZFOURGE provides well-constrained photometric redshifts made
possible through deep medium-bandwidth imaging at 1-2um . We combine this with
HST imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey
(CANDELS), allowing for the efficient selection of both blue and red galaxies
down to stellar masses ~10^9.5 Msol at z ~ 2.5. The total surveyed area is 316
arcmin^2 distributed over three independent fields. We supplement these data
with the wider and shallower NEWFIRM Medium-Band Survey (NMBS) to provide
stronger constraints at high masses. Several studies at z<=1 have revealed a
steepening of the slope at the low-mass end of the stellar mass function (SMF),
leading to an upturn at masses <10^10 Msol that is not well-described by a
standard single-Schechter function. We find evidence that this feature extends
to at least z ~ 2, and that it can be found in both the star-forming and
quiescent populations individually. The characteristic mass (M*) and slope at
the lowest masses (alpha) of a double-Schechter function fit to the SMF stay
roughly constant at Log(M/Msol) ~ 10.65 and ~-1.5 respectively. The SMF of
star-forming galaxies has evolved primarily in normalization, while the change
in shape is relatively minor. This is not the case for quiescent galaxies: the
depth of our imaging allows us to show for the first time significantly more
evolution at Log(M/Msol) < 10.5 than at higher masses. We find that the total
mass density (down to 10^9 Msol) in star-forming galaxies has increased by a
factor of ~2.2 since z ~ 2.5, whereas in quiescent galaxies it has increased by
a factor of ~12 .
