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

Color and number counts

¹ Osservatorio Astronomico di Brera, via E. Bianchi 46, Merate, Italy
² Osservatorio Astronomico di Roma, via Dell'Osservatorio 2, Monteporzio, Italy
³ Space Telescope European Coordinating Facility, ESO, Karl-Schwarzschildstr. 2, Garching bei Munchen, Germany
⁴ Dipartimento di Astronomia, Universitá di Padova, Vicolo dell'Osservatorio, 2, Padova, Italy
⁵ ESO, Karl-Schwarzschildstr. 2, Garching bei Munchen, Germany

Corresponding author: P. Saracco, saracco@merate.mi.astro.it

Received: 20 December 2000
Accepted: 4 April 2001

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 arcmin². The limiting surface brightness obtained is mag/arcsec² and (1σ) on both fields. A d /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 color of galaxies becomes bluer at magnitudes fainter than , 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 at magnitudes . Most of them appear as isolated sources, possibly elliptical or dusty starburst galaxies at redshift . 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 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.