2010ApJ...725.1277M

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 M_star>2.5x10¹¹ M_☉, 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 <r_tot> = 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 M_☉/yr 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)x10¹³ L_☉. This implies the presence of either dust-enshrouded starburst activity (with SFRs of 600-4300 M_☉/yr) and/or highly obscured active galactic nuclei (AGNs). The contribution of galaxies with M_star>2.5x10¹¹ M_☉ 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 M_star~ 10⁹-10¹¹ M_☉ 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.