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 ~ 1.5 based on a spectroscopic survey of a magnitude limited sample of galaxies with K_s<20 (the K20 survey, Cimatti et al. \cite{Cimatti2002b}). 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 =~ 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_{K_s}-5 log h₇₀<-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 =~ 1.5, with an amplitude of about Delta M_J =~ -0.69+/-0.12 and Delta M_K =~ -0.54+/-0.12 at z ~ 1. Pure density evolution is not consistent with the observed LF at zle1 . 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 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 =~ 1.5, finding a slow evolution with redshift (rho_lambda (z)= rho_lambda (z=0) (1+z)^{beta (lambda )} with beta (J) =~ 0.70 and beta (K_s) =~ 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 zle1 and underpredict the density of luminous galaxies at zge1 , 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.

Based on observations made at the European Southern Observatory, Chile (ESO LP 164.O-0560).