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 mu m 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 mu m band to ensure that the near-IR 1.6 mu m (rest-frame) peak is falling in or beyond this band. Results. We found 53 z = 3.5 candidates, of masses in the range M-star similar to 10(10)-10(11) M-circle dot. At least similar to 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 x 10(10) M-circle dot (a reasonable stellar-mass completeness limit for our sample) is 2.6 x 10(-5) Mpc(-3) (using the volume within 3.5 < z < 5), and the corresponding stellar mass density is similar to(2.9 +/- 1.5) x 10(6) M-circle dot 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 similar to 0.97 x 10(-5) Mpc(-3) and a stellar mass density of similar to(1.15 +/- 0.7) x 10(6) M-circle dot 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 similar to 15 between z similar to 4 and z = 2, and by similar to 100 to z = 0.