The K20 survey. V. The evolution of the near-IR Luminosity Function

We present the galaxy rest-frame near-IR Luminosity Function (LF) and its cosmic evolution to z similar to 1.5 based on a spectroscopic survey of a magnitude limited sample of galaxies with K-s < 20 ( the K20 survey, Cimatti et al. 2002b). The LFs have been derived in the rest-frame J and K-s bands. Their evolution is traced using three different redshift bins (z(mean) similar or equal to 0.5; 1, 1.5) and comparing them to the Local near-IR Luminosity Function. The luminosity functions at different redshifts are fairly well fitted by Schechter functions 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 (M-Ks - 5 log h(70) < - 25.5) is higher than locally at all redshifts 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 K-s-band up to z similar or equal to 1.5, with an amplitude of about Delta M-J similar or equal to -0.69 +/- 0.12 and Delta M-K similar or equal to - 0.54 +/- 0.12 at z similar to 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 similar or equal to 1, thus suggesting that massive elliptical galaxies were already in place at z similar or equal to 1 and they should have formed their stars and assembled their mass at higher 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 similar or equal to 1.5, finding a slow evolution with redshift (rho(lambda)(z) = rholambda(z = 0)(1 + z)(beta(lambda)) with beta(J) similar or equal to 0.70 and beta(K-s) similar or equal to 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 less than or equal to 1 and underpredict the density of luminous galaxies at z greater than or equal to 1, whereas passive evolution models are more consistent with the data up to z similar to 1.5. The GIF model (Kaufmann et al. 1999) shows a clear deficiency of red luminous galaxies at z similar to 1 compared to our observations and predicts a decrease of luminous galaxies with redshift not observed in our sample.