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

Free access article

Issue		A&A Volume 375, Number 1, August III 2001


Page(s)		1 - 13
Section		Cosmology
DOI		http://dx.doi.org/10.1051/0004-6361:20010795

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

Color and number counts

P. Saracco¹, E. Giallongo², S. Cristiani^{3, 4}, S. D'Odorico⁵, A. Fontana², A. Iovino², F. Poli² and E. Vanzella^{5, 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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