Astronomy & Astrophysics

Free access article

Issue		A&A Volume 420, Number 1, June II 2004
Page(s)		125 - 133
Section		Extragalactic astronomy
DOI		10.1051/0004-6361:20040101

A&A 420, 125-133 (2004)
DOI: 10.1051/0004-6361:20040101

High-z massive galaxies in the Hubble Deep Field South

P. Saracco¹, M. Longhetti¹, E. Giallongo², S. Arnouts³, S. Cristiani⁴, S. D'Odorico⁵, A. Fontana², M. Nonino⁴ and E. Vanzella^{5, 6}

¹ INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    e-mail: marcella@brera.mi.astro.it
² INAF - Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monte Porzio Catone, Italy
    e-mail: [giallo, fontana]@coma.mporzio.astro.it
³ Laboratoire d'Astronomie de Marseille, Traverse du Siphon, BP 8, 13376 Marseille, France
    e-mail: stephane.arnouts@oamp.fr
⁴ INAF - Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 40131 Trieste, Italy
    e-mail: [cristiani, nonino]@ts.astro.it
⁵ European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching, Germany
    e-mail: [sdodoric, evanzell]@eso.org
⁶ Dipartimento di Astronomia dell'Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy

(Received 16 May 2003 / Accepted 25 February 2004 )

Abstract
A census of massive galaxies at redshift increasingly higher than $z\sim1$ may provide strong constraints for the history of mass assembly and star formation. Here we report the analysis of three galaxies selected in the Hubble Deep Field South at $K {\rm s}\le22$ on the basis of their unusually red near-IR color $J-K\ge3$ . We have used population synthesis models to constrain their redshifts and their stellar masses. One galaxy (HDFS-1269) is at redshift $z_{\rm phot}\simeq2.4$ while the other two (HDFS-822 and HDFS-850) are at $z_{\rm phot}\simeq2.9{-}3.0$ . All three galaxies have already assembled a stellar mass of about 10¹¹ $M_{\odot}$ at the observed redshift, placing the possible merging event of their formation at $z\ga 3.5$ . The inferred mass weighted age of their stellar populations implies that the bulk of the stars formed at $z_{\rm f}>3.5$ . The resulting co-moving density of $\mathcal{M}_{\rm stars}\ga 10^$ $M_{\odot}$ galaxies at $\langle z\rangle\simeq2.7$ is $\rho=1.2 \pm\,0.7\times 10^$ Mpc ^-3, about a factor two higher than the predictions of hierarchical models. The comparison with the local density of galaxies implies that the three galaxies must have already formed most of their stellar mass and that they cannot follow an evolution significantly different from a passive aging.

The comparison with the density of local $L\ge L^*$ early types (passively evolved galaxies) suggests that their co-moving density cannot decrease by more than a factor 2.5-3 from z=0 to $z\simeq3$ suggesting that up to 40% of the stellar mass content of bright ( $L\ge L^*$ ) local early type galaxies was already in place at z>2.5.

Key words: galaxies: evolution -- galaxies: elliptical and lenticular, cD -- galaxies: formation

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

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