SPITZER BRIGHT, ULTRAVISTA FAINT SOURCES IN COSMOS: THE CONTRIBUTION TO THE OVERALL POPULATION OF MASSIVE GALAXIES AT z = 3-7

Abstract

We have analyzed a sample of 574 Spitzer 4.5 μm selected galaxies with [4.5] < 23 and Ksauto > 24 (AB) over the UltraVISTA ultradeep COSMOS field. Our aim is to investigate whether these mid-infrared (mid-IR) bright, near-infrared (near-IR) faint sources contribute significantly to the overall population of massive galaxies at redshifts z ≥ 3. By performing a spectral energy distribution (SED) analysis using up to 30 photometric bands, we have determined that the redshift distribution of our sample peaks at redshifts z ≈ 2.5-3.0, and ∼32% of the galaxies lie at z ≥ 3. We have studied the contribution of these sources to the galaxy stellar mass function (GSMF) at high redshifts. We found that the [4.5] < 23, Ksauto > 24 galaxies produce a negligible change to the GSMF previously determined for Ksauto < 24 sources at 3 ≤ z < 4, but their contribution is more important at 4 ≥ z < 5, accounting for ≳50% of the galaxies with stellar masses Mst ≳ 6 × 1010 M⊙. We also constrained the GSMF at the highest-mass end (Mst ≳ 2 × 1011 M⊙) at z ≥ 5. From their presence at 5 ≤ z < 6 and virtual absence at higher redshifts, we can pinpoint quite precisely the moment of appearance of the first most massive galaxies as taking place in the ∼0.2 Gyr of elapsed time between z ∼ 6 and z ∼ 5. Alternatively, if very massive galaxies existed earlier in cosmic time, they should have been significantly dust-obscured to lie beyond the detection limits of current, large-area, deep near-IR surveys.