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. The limiting surface brightness obtained
is Ks=22.8 mag/arcsec^2 and
J=24.5 (1sigma)
on both fields. A dlog(N)/dm relation with a slope of ~0.34 in J and ~0.28
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~18, in agreement with the
different number
counts slope observed in the two bands. We find 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 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.