Deep near-IR observations of the Chandra Deep Field and of the HDF South - Color and number counts

We present near-IR (J and Ks) number counts and colors of galaxies detected in deep VLT-ISAAC images centered on the Chandra Deep Field and Hubble Deep Field-South for a total area of 13.6 arcmin(2). The limiting surface brightness obtained is Ks similar or equal to 22.8 mag/arcsec(2) and J similar or equal to 24.5 (1 sigma )on both fields.A d log N/dm relation with a slope of similar to0.34 in J and similar to0.28 in Ks is found in both fields with no evidence of decline near the magnitude limit. The median J Ks color of galaxies becomes bluer at magnitudes fainter than Ks similar to 18, in agreement with the different number counts slope observed in the two bands. We find a fraction (less than or equal to5% of the total sample) of sources with color redder than J-Ks = 2.3 at magnitudes Ks > 20. Most of them appear as isolated sources, possibly elliptical or dusty starburst galaxies at redshift z >2. The comparison of the observed number counts with models shows that our J-band and Ks-band counts are consistent with the prediction of a model based on a small amount of merging in a Omega = 1 cosmology. On the other hand, we fail to reproduce the observed counts if we do not consider merging independently of the parameters defining the universe.