PHYSICAL PROPERTIES OF SPECTROSCOPICALLY CONFIRMED GALAXIES AT z >= 6. III. STELLAR POPULATIONS FROM SED MODELING WITH SECURE Ly alpha EMISSION AND REDSHIFTS

We present a study of stellar populations in a sample of spectroscopically confirmed Lyman-break galaxies (LBGs) and Ly alpha 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 Ly alpha 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 similar to 10(8) to similar to 10(11)M(circle dot). 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-circle dot, and a "young" subsample consisting of galaxies younger than similar to 30 Myr, with masses ranging between similar to 10(8) and similar to 3 x 10(9) M-circle dot. 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 >= 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 Ly alpha emission are indistinguishable in terms of age, stellar mass, and SFR.985 project at Peking University; NASA, STScI [11149, 12329, 12616]; JPL/Caltech [40026, 70094]SCI(E)ARTICLEjiangKIAA@pku.edu.cn181