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Issue A&A
Volume 375, Number 1, August III 2001
Page(s) 1 - 13
Section Cosmology
DOI 10.1051/0004-6361:20010795



A&A 375, 1-13 (2001)
DOI: 10.1051/0004-6361:20010795

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

Color and number counts
P. Saracco1, E. Giallongo2, S. Cristiani3, 4, S. D'Odorico5, A. Fontana2, A. Iovino2, F. Poli2 and E. Vanzella5, 4

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

(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 ${\it Ks}\simeq22.8$ mag/arcsec2 and $J\simeq24.5$ (1$\sigma$) on both fields. A d $\log N$/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 $J-{\it Ks}$ color of galaxies becomes bluer at magnitudes fainter than ${\it Ks}\sim18$, in agreement with the different number counts slope observed in the two bands. We find a fraction ($\le$5% of the total sample) of sources with color redder than $J-{\it Ks}=2.3$ at magnitudes ${\it Ks}> 20$. 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.


Key words: cosmology: large scale structure of the Universe -- galaxies: general

Offprint request: P. Saracco, saracco@merate.mi.astro.it

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© ESO 2001


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