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

Creators: Berta, S.; Lonsdale, C. J.; Polletta, M.; Savage, R. S.; Franceschini, A.; Buttery, H.; Cimatti, A.; Dias, J.; Feruglio, C.; Fiore, F.; Held, E. V.; La Franca, F.; Maiolino, R.; Marconi, A.; Matute, I.; Oliver, S. J.; Ricciardelli, E.; Rubele, S.; Sacchi, N.; Shupe, D.; Surace, J.

Abstract

Context. In the last couple of years a population of very massive (M_* > 10^(11) M_⊙), 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 deg^2). Galaxies with the 1.6 μm stellar peak redshifted into the IRAC bands (z ≃ 1−3, called "IR-peakers") were identified. Stellar masses were derived by means of spectrophotometric 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 z = 2 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 10^(11) M_⊙ 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 M_*/L 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 10^(11) and ~10^(12) M_⊙ is probed with unprecedented detail. A significant evolution is found not only for galaxies with M ~ 10^(11) M_⊙, 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 × 10^(11) M_⊙ provide 30−50% of the total stellar mass density in galaxies at z = 2−3.

Additional Information

© ESO 2007. Received 16 March 2007. Accepted 24 September 2007 We also would like to thank the whole SWIRE team for preparing the ES1 Spitzer data. We are grateful to Stephan Charlot for very useful discussions about SED fitting, IMF, and AGB stars, and Paolo Cassata on the stellar mass function and units. Finally, SB wishes to thank people at the Sussex Astronomy Center for their warm hospitality. This work made use of observations collected at the European Southern Observatory, Chile: ESO projects No. 168.A-0322, 170.A-0143, 073.A-0446, 075.A-0428. The Spitzer Space Telescope is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. SWIRE was supported by NASA through the SIRTF Legacy Program under contract 1407 with the Jet Propulsion Laboratory.

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