Searching for massive galaxies at z≥ 3.5 in GOODS-North

Aims: 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. Methods: The z≥3.5 sample was selected to m_AB=23 mag at 4.5 μm 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 8.0 μm band to ensure that the near-IR 1.6 μm (rest-frame) peak is falling in or beyond this band. Results: We found 53 z ≥3.5 candidates, of masses in the range Mstar˜1010{-}1011 Msun. 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 5 × 1010 Msun (a reasonable stellar-mass completeness limit for our sample) is 2.6 × 10-5 Mpc-3 (using the volume within 3.5<z<5), and the corresponding stellar mass density is ˜(2.9± 1.5) × 10^6 Msun Mpc-3, 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 ˜0.97 × 10-5 Mpc-3 and a stellar mass density of ˜(1.15± 0.7) × 10^6 Msun Mpc-3. 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.