A Significant Population of Red, Near-Infrared-selected High-Redshift Galaxies

We use very deep near-infrared photometry of the Hubble Deep Field Southtaken with ISAAC/VLT to identify a population of high redshift galaxies withrest- frame optical colors similar to those of nearby galaxies. The galaxiesare chosen by their infrared colors Js-Ks > 2.3, aimed at selecting galaxieswith redshifts above 2. When applied to our dataset, we find 14 galaxies withKs < 22.5, corresponding to a surface density of 3+-0.8 /arcmin**2. Thephotometric redshifts all lie above 1.9, with a median of 2.6 and a rms of 0.7.The spectral energy distributions of these galaxies show a wide range: one isvery blue in the rest-frame UV, and satisfies the normal Lyman-break criteriafor high redshift, star-forming galaxies. Others are quite red throughout theobserved spectral range, and are extremely faint in the optical, with a medianV = 26.6. Hence these galaxies would not be included in photometric samplesbased on optical ground-based data, and spectroscopic follow-up is difficult.The spectral energy distributions often show a prominent break, identified asthe Balmer break or 4000 Ang. break. The median age is 1 Gyr when fit with aconstant star formation model with dust, or 0.7 Gyr when fit with a singleburst model. Although significantly younger ages cannot be excluded when alarger range of models is allowed, the results indicate that these galaxies areamong the oldest at these redshifts. The volume density to Ks=22.5 is half thatof Lyman-break galaxies at z = 3. Since the mass-to-light ratios of the redgalaxies are likely to be higher, the stellar mass density is inferred to becomparable to that of Lyman-break galaxies. These red galaxies may be thedescendants of galaxies which started to form stars at very high redshifts, andthey may evolve into the most massive galaxies at low redshift.Comment: LaTex, 5 pages and 4 figures. Accepted for publication in ApJ Letters. See also related preprints on astroph today by Daddi et al and van Dokkum et a