The K20 survey. V. The evolution of the near-IR luminosity function

Abstract

We present the galaxy rest-frame near-IR Luminosity Function (LF) and its cosmic evolution to z ∼ 1.5 based on a spectroscopic survey of a magnitude limited sample of galaxies with Ks < (the K20 survey, Cimatti et al. 2002b). The LFs have been derived in the rest-frame J and Ks bands. Their evolution is traced using three different redshift bins (zmean ≃ 0.5, 1, 1.5) and comparing them to the Local near-IR Luminosity Function. The luminosity functions at different redshisfts are fairly well fitted by Schechter function at z < 1.3. The faint-end of the LFs (L < L*) is consistent with the local estimates, with no evidence for a change either in the slope or normalization up to z < 1.3. At higher redshift this part of the luminosity function, is not well sampled by our data. Viceversa, the density of luminous galaxies (MKs log h70 < -25.5) is higher than locally at all redshift and relatively constant or mildly increasing with redshift within our sample. The data are consistent with a mild luminosity evolution both in the J- and Ks-band up to z ≃ 1.5, with an amplitude of about ΔMJ ≃ 0.69 ≃ 0.12 and ΔMK ≃ -0.54 ± 0.12 at z ∼ 1. Pure density evolution is not consistent with the observed LF at z ≤ 1. Moreover, we find that red and early-type galaxies dominate the bright-end of the LF, and that their number density shows at most a small decrease (>30%) up to z ≃ 1, thus suggesting that massive elliptical galaxies were already in place at z ≃ 1 and they should have formed their stars and assembled their mass at high redshift. There appears to be a correlation of the optical/near-IR colors with near-IR luminosities, the most luminous/massive galaxies being red/old, the low-luminous galaxies being instead dominated by blue young stellar populations. We also investigate the evolution of the near-IR comoving luminosity density to z ≃ 1.5, finding a slow evolution with redshift (ρλ(z) = ρλ(z = 0)(1 + z)β(λ) with β(J) ≃ 0.70 and β(Ks) ≃ 0.37). Finally, we compare the observed LFs with the predictions of a set of the most updated hierarchical merging models. Such a comparison shows that the current versions of hierarchical models overpredict significantly the density of low luminosity galaxies at z ≤ 1 and underpredict the density of luminous galaxies at z ≥ 1, whereas passive evolution models are more consistent with the data up to z ∼ 1.5. The GIF model (Kaufmann et al. 1999) shows a clear deficiency of red luminous galaxies at z ∼ 1 compared to our observations and predicts a decrease of luminous galaxies with redshift not observed in our sample.