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$\simeq$22.8 mag/arcsec$^2$ and J$\simeq$24.5 (1$\sigma$) on both fields. A d$log$N/dm relation with a slope of $\sim0.34$ in J and $\sim0.28$ 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$\sim18$, in agreement with the different number counts slope observed in the two bands. We find a fraction ($\le5%$ 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