Spitzer bright, UltraVISTA faint sources in COSMOS: the contribution to the overall population of massive galaxies at z = 3-7

Abstract

We have analysed a sample of 574 Spitzer 4.5 μm-selected galaxies with [4.5] < 23 and K_s^(auto) > 24 (AB) over the UltraVISTA ultra-deep COSMOS field. Our aim is to investigate whether these mid-IR bright, 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, K_s^(auto) > 24 galaxies produce a negligible change to the GSMF previously determined for K_s^(auto) < 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 M_(st) ≳ 6 × 10^(10)M⊙. We also constrained the GSMF at the highest-mass end (M_(st) ≳ 2 × 10^(11)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 ~ 0.2Gyr 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.

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