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

Context. In the last couple of years a population of very massive (M > 1011 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 deg2). 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 1011 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 1011 and ∼1012 M is probed with unprecedented detail. A significant evolution is found not only for galaxies with M ∼ 1011 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 × 1011 M provide 30−50% of the total stellar mass density in galaxies at z = 2−3.