Possible identification of massive and evolved galaxies at z ≥ 5

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 z5. 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±0.2M yr1, dust AV < 1, and age =0.7 ± 0.4Gyr at z = 5.7 ± 0.6, where the dusty star-forming galaxies at z 2.8 are disfavored because of the faintness in the 24m. The stellar mass density of these evolved galaxy candidates, (6±4) 104M Mpc3, 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.