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

Abstract

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 .T he limiting surface brightness obtained is Ks ' 22:8 mag/arcsec 2 and J ' 24: 5( 1 ) on bothelds. A d logN/dm relation with a slope of0.34 in J and0.28 in Ks is found in both elds with no evidence of decline near the magnitude limit. The median J Ks color of galaxies becomes bluer at magnitudes fainter than Ks 18, in agreement with the dierent number counts slope observed in the two bands. We nd a fraction (5% 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 = 1 cosmology. On the other hand, we fail to reproduce the observed counts if we do not consider merging independently of the parameters dening the universe.