The contribution of very massive high-redshift SWIRE galaxies to the stellar mass function

¹ Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 3, 35122 Padova, Italy e-mail: berta@mpe.mpg.de
² Center for Astrophysics and Space Sciences, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0424, USA
³ Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
⁴ Infrared Processing & Analysis Center, California Institute of Technology 100-22, Pasadena, CA 91125, USA
⁵ Institut d'Astrophysique de Paris, 98bis bld Arago, 75014 Paris, France
⁶ Astronomy Centre, CPES, University of Sussex, Falmer, Brighton BN19QJ, UK
⁷ INAF – Osservatorio Astronomico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00044 Monteporzio Catone, Italy
¹⁰ INAF – Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
¹¹ Dipartimento di Fisica, Università degli Studi “Roma Tre”, Via della Vasca Navale 84, 00146 Roma, Italy
¹² Dipartimento di Astronomia e Scienza dello Spazio, Università di Firenze, Largo E. Fermi 2, 50125 Firenze, Italy
¹³ Spitzer Science Center, California Institute for Technology, 220-6, Pasadena, CA 91125, USA

Received: 16 March 2007
Accepted: 24 September 2007

Abstract

Context.In the last couple of years a population of very massive (>10¹¹ ), high-redshift (z≥2) galaxies has been identified, but its role in galaxy evolution has not yet been fully understood.

Aims.It is necessary to perform a systematic study of high-redshift massive galaxies, in order to determine the shape of the very massive tail of the stellar mass function and determine the epoch of their assembly.

Methods.We selected high-z massive galaxies at 5.8 μm, in the SWIRE ELAIS-S1 field (1 deg²). Galaxies with the 1.6 μm stellar peak redshifted into the IRAC bands (z1-3, called “IR-peakers”) were identified. Stellar masses were derived by means of spectro-photometric fitting and used to compute the stellar mass function (MF) at z = 1-2 and 2-3. A parametric fit to the MF was performed, based on a Bayesian formalism, and the stellar mass density of massive galaxies above determined.

Results.We present the first systematic study of the very-massive tail of the galaxy stellar mass function at high redshift. A total of 326 sources were selected. The majority of these galaxies have stellar masses in excess of 10¹¹ and lie at z > 1.5. The availability of mid-IR data turned out to be a valuable tool to constrain the contribution of young stars to galaxy SEDs, and thus their  ratio. The influence of near-IR data and of the chosen stellar library on the SED fitting are also discussed. The z = 2-3 stellar mass function between 10¹¹ and ~10¹² is probed with unprecedented detail. A significant evolution is found not only for galaxies with M ~10¹¹ , but also in the highest mass bins considered. The comoving number density of these galaxies was lower by more than a factor of 10 at z = 2-3, with respect to the local estimate. SWIRE 5.8 μm peakers more massive than 1.6 10¹¹ provide 30-50% of the total stellar mass density in galaxies at z = 2-3.