Near-Infrared Bright Galaxies at z ≃ 2. Entering the Spheroid Formation Epoch?^*

Spectroscopic redshifts have been measured for nine K-band luminous galaxies at 1.7 < z < 2.3, selected with K_s < 20 in the K20 survey region of the Great Observatories Origins Deep Survey (GOODS) area. Star formation rates (SFRs) of ~100-500 M_☉ yr^-1 are derived when dust extinction is taken into account. The fitting of their multicolor spectral energy distributions indicates stellar masses of M ≳ 10¹¹ M_☉ for most of the galaxies. Their rest-frame UV morphology is highly irregular, suggesting that merging-driven starbursts are going on in these galaxies. Morphologies tend to be more compact in the near-IR, a hint for the possible presence of older stellar populations. Such galaxies are strongly clustered, with seven out of nine belonging to redshift spikes, which indicates a correlation length of r₀ ~ 9-17 h^-1 Mpc (1 σ range). Current semianalytical models of galaxy formation appear to underpredict by a large factor (≳30) the number density of such a population of massive and powerful starburst galaxies at z ~ 2. The high masses and SFRs, together with the strong clustering, suggest that at z ~ 2 we may have started to explore the major formation epoch of massive early-type galaxies.