We present a study of stellar populations in a sample of spectroscopically confirmed Lyman-break galaxies (LBGs) and Lyα emitters (LAEs) at 5.7< z< 7. These galaxies have deep images from Subaru, the Hubble Space Telescope, and Spitzer/IRAC. We focus on 27 galaxies with IRAC detections, and characterize their stellar populations based on the multi-band data and secure redshifts. By estimating nebular emission from the observed Lyα flux, we break the strong model degeneracy between young galaxies with prominent nebular emission and older galaxies with strong Balmer breaks. The results show that our galaxies cover a wide range of ages from several to a few hundred million years (Myr), and stellar masses from ∼10⁸ to ∼10{}¹¹ {M}_⊙ . These galaxies can be roughly divided into two subsamples: an “old” subsample consisting of galaxies older than 100 Myr, with stellar masses higher than {10}⁹ {M}_⊙ , and a “young” subsample consisting of galaxies younger than ∼30 Myr, with masses ranging between ∼10⁸ and ∼ 3× {10}⁹ {M}_⊙ . Both subsamples display a correlation between stellar mass and star formation rate (SFR), but with very different normalizations. The average specific SFR (sSFR) of the “old” subsample is 3-4 Gyr^-1, consistent with previous studies of “normal” star-forming galaxies at z≥slant 6. The average sSFR of the “young” subsample is an order of magnitude higher, likely due to starburst activity. Our results also indicate little dust extinction in the majority of the galaxies, as already suggested by their steep rest-frame UV slopes. Finally, LAEs and LBGs with strong Lyα emission are indistinguishable in terms of age, stellar mass, and SFR.

Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Based in part on data collected at Subaru Telescope and obtained from the SMOKA, which is operated by the Astronomy Data Center, National Astronomical Observatory of Japan.