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 < 20 (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 dierent redshift bins (zmean ' 0:5; 1; 1:5) and comparing them to the Local near-IR Luminosity Function. The luminosity functions at dierent 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 ( MKs 5l ogh70< 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 evolutionboth in the J -a ndKs -band up toz' 1:5, with an amplitude of aboutMJ ' 0:69 0:12 and MK ' 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' 1a ndthey 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 ((z)=(z= 0)(1+z) () with(J)' 0:70 and(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.