2009A&A...500..705M

We constrain the space density and properties of massive galaxy candidates at redshifts of z≥3.5 in the Great Observatories Origin Deep Survey North (GOODS-N) field. By selecting sources in the Spitzer + IRAC bands, a sample highly complete in stellar-mass is assembled, including massive galaxies that are very faint in the optical/near-IR bands and would be missed by samples selected at shorter wavelengths. The z≥3.5 sample was selected to m_AB=23mag at 4.5µm using photometric redshifts obtained by fitting the galaxies spectral energy distribution at optical, near-IR bands, and IRAC bands. We also require that the brightest band (in AB scale) in which candidates are detected is the IRAC 8.0µm band to ensure that the near-IR 1.6µm (rest-frame) peak is falling in or beyond this band. We found 53 z≥3.5 candidates, of masses in the range M_*∼10¹⁰-10¹¹M_☉. At least ∼81% of these galaxies are missed by traditional Lyman Break selection methods based on ultraviolet light. Spitzer + MIPS emission is detected for 60% of the sample of z≥3.5 galaxy candidates. Although in some cases this might suggest a residual contamination from lower redshift star-forming galaxies or Active Galactic Nuclei, 37% of these objects are also detected in the sub-mm/mm bands in SCUBA, AzTEC, and MAMBO surveys, and have properties fully consistent with vigorous starburst galaxies at z≥3.5. The comoving number density of galaxies with stellar masses of above 5x10¹⁰ M_☉ (a reasonable stellar-mass completeness limit for our sample) is 2.6x10^–5/Mpc3 (using the volume within 3.5<z<5), and the corresponding stellar mass density is ~(2.9±1.5)x10⁶M_☉/Mpc³, or about 3% of the local density above the same stellar mass limit. For the subsample of MIPS-undetected galaxies, we measure a number density of ∼0.97x10^–5Mpc^–3 and a stellar mass density of ~(1.15±0.7)x10⁶M_☉/Mpc³. Even in the unlikely case that these are all truly quiescent galaxies, this would imply an increase in the space density of passive galaxies by a factor of ∼15 between z∼4 and z=2, and by ∼100 to z=0.