Red-sequence galaxies at high redshift by the COMBO-17+4 survey

We investigate the evolution of the galaxy population since redshift 2 with a focus on the color bimodality and mass density of the red sequence. We obtain precise and reliable photometric redshifts up to z = 2 by supplementing the optical survey COMBO-17 with observations in four near-infrared bands on 0.2 deg² of the COMBO-17 A901-field. Our results are based on an H-band-selected catalog of 10,692 galaxies complete to H = 21^m.7. We measure the rest-frame color (U₂₈₀-V) of each galaxy, which across the redshift range of our interest requires no extrapolation and is robust against moderate redshift errors by staying clear of the 4000 Å break. We measure the color-magnitude relation of the red sequence as a function of look-back time from the peak in a color-error-weighted histogram, and thus trace the galaxy bimodality out to z ≳ 1.65. The (U_280-V) of the red sequence is found to evolve almost linearly with look-back time. At high redshift, we find massive galaxies in both the red and the blue population. Red-sequence galaxies with log M_*/M_⊙ > 11 increase in mass density by a factor of ∼4 from z ∼ 2 to 1 and remain nearly constant at z < 1. However, some galaxies as massive as log M _*/M_⊙ = 11.5 are already in place at z ∼ 2.