Possible identification of massive and evolved galaxies at z ≳ 5

Abstract

Abstract We report on the identification of the old stellar population galaxy candidates at z ≳ 5. We developed a new infrared color selection scheme to isolate galaxies with the strong Balmer breaks at z ≳ 5, and applied it to the ultra-deep and wide infrared survey data from the Spitzer Extended Deep Survey (SEDS) and the UKIRT Infrared Deep Sky Survey. The eight objects satisfying K − [3.6] > 1.3 and K − [3.6] > 2.4([3.6] − [4.5]) + 0.6 are selected in the 0.34 deg2 SEDS Ultra Deep Survey field. Rich multi-wavelength imaging data from optical to far-infrared are also used to reject blending sources and strong nebular line emitters, and we finally obtained the three most likely evolved galaxies at z ≳ 5. Their stacked spectral energy distribution is fitted well with the old stellar population template with M* = (7.5 ± 1.5) × 1010 M⊙, star formation rate = 0.9 ± 0.2 M⊙ yr−1, dust AV < 1, and age =0.7 ± 0.4 Gyr at z = 5.7 ± 0.6, where the dusty star-forming galaxies at z ∼ 2.8 are disfavored because of the faintness in the 24 μm. The stellar mass density of these evolved galaxy candidates, (6 ± 4) × 104 M⊙ Mpc−3, is much lower than that of star-forming galaxies, but the non-zero fraction suggests that initial star-formation and quenching have been completed by z ∼ 6.