Galaxy stellar mass functions from zFourGE/CANDELS : an excess of low-mass galaxies since z=2 and the rapid buildup of quiescient galaxies

Using observations from the FourStar Galaxy Evolution Survey (ZFOURGE), we obtain the deepest measurements to date of the galaxy stellar mass function (SMF) at 0.2 < z < 3. ZFOURGE provides well-constrained photometric redshifts made possible through deep medium-bandwidth imaging at 1-2 mu m. We combine this with Hubble Space Telescope imaging from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey, allowing for the efficient selection of both blue and red galaxies down to stellar masses of similar to 10(9.5)M(circle dot) at z similar to 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 to provide stronger constraints at high masses. Several studies at z <= 1.5 have revealed a steepening of the slope at the low-mass end of the SMF, leading to an upturn at masses < 10(10)M(circle dot) that is not well described by a standard single-Schechter function. We find evidence that this feature extends to at least z similar to 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/M-circle dot) similar to 10.65 and similar to-1.5, respectively. The SMF of star-forming galaxies has evolved primarily in normalization, while the change in shape is relatively minor. Our data allow us, for the first time, to observe a rapid buildup at the low-mass end of the quiescent SMF. Since z = 2.5, the total stellar mass density of quiescent galaxies (down to 10(9)M(circle dot)) has increased by a factor of similar to 12, whereas the mass density of star-forming galaxies only increases by a factor of similar to 2.2