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

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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.

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