THE MOST MASSIVE GALAXIES AT 3.0 ≤ z < 4.0 IN THE NEWFIRM MEDIUM-BAND SURVEY: PROPERTIES AND IMPROVED CONSTRAINTS ON THE STELLAR MASS FUNCTION

We use the optical to mid-infrared coverage of the NEWFIRM Medium-Band Survey (NMBS) to characterize, for the first time, the properties of a mass-complete sample of 14 galaxies at 3.0 ≤ z < 4.0 with Mstar>2.5 x 1011 Msun, and to derive significantly more accurate measurements of the high-mass end of the stellar mass function (SMF) of galaxies at 3.0 ≤ z < 4.0. The accurate photometric redshifts and well-sampled spectral energy distributions (SEDs) provided by the NMBS combined with the large surveyed area result in significantly reduced contributions from photometric redshift errors and cosmic variance to the total error budget of the SMF. The typical very massive galaxy at 3.0 ≤ z < 4.0 is red and faint in the observer's optical, with a median r-band magnitude of (rtot) = 26.1, and median rest-frame U - V colors of (U - V) = 1.6. About 60% of the mass-complete sample has optical colors satisfying either the U- or the B-dropout color criteria, although ∼50% of these galaxies has r>25.5. We find that ∼30% of the sample has star formation rates (SFRs) from SED modeling consistent with zero, although SFRs of up to ∼1-18 Msun yr-1 are also allowed within 1σ. However, >80% of the sample is detected at 24 μm, resulting in total infrared luminosities in the range (0.5-4.0) x 1013 Lsun. This implies the presence of either dust-enshrouded starburst activity (with SFRs of 600-4300 Msun yr-1) and/or highly obscured active galactic nuclei (AGNs). The contribution of galaxies with Mstar>2.5 x 1011 Msun to the total stellar mass budget at 3.0 ≤ z < 4.0 is ∼8+13-3%. Compared to recent estimates of the stellar mass density in galaxies with Mstar ∼ 109-1011 Msun at z ∼ 5 and z ∼ 6, we find an evolution by a factor of 2-7 and 3-22 from z ∼ 5 and z ∼ 6, respectively, to z = 3.5. The previously found disagreement at the high-mass end between observed and model-predicted SMFs is now significant at the 3σ level when only random uncertainties are considered. However, systematic uncertainties dominate the total error budget, with errors up to a factor of ∼8 in the densities at the high-mass end, bringing the observed SMF in marginal agreement with the predicted SMF. Additional systematic uncertainties on the high-mass end could be potentially introduced by either (1) the intense star formation and/or the very common AGN activities as inferred from the MIPS 24 μm detections, and/or (2) contamination by a significant population of massive, old, and dusty galaxies at z ∼ 2.6.