STAR FORMATION RATES AND STELLAR MASSES OF z = 7–8 GALAXIES FROM IRAC OBSERVATIONS OF THE WFC3/IR EARLY RELEASE SCIENCE AND THE HUDF FIELDS

We investigate the Spitzer /IRAC properties of 36 z ∼ 7 z 850 -dropout galaxies and three z ∼ 8 Y 098 galaxies derived from deep/wide-area WFC3/IR data of the Early Release Science, the ultradeep HUDF09, and wide-area NICMOS data. We fit stellar population synthesis models to the spectral energy distributions to derive mean redshifts, stellar masses, and ages. The z ∼ 7 galaxies are best characterized by substantial ages (>100 Myr) and M / L V ≈ 0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from β ∼ −2.0 to β ∼ −3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1 L * z = 3 ), where low metallicity and smooth star formation histories (SFHs) fail to match the blue far-UV and moderately red H − [3.6] color. Such colors may require episodic SFHs with short periods of activity and quiescence ("on–off" cycles) and/or a contribution from emission lines. The stellar mass of our sample of z ∼ 7 star-forming galaxies correlates with star formation rate (SFR) according to log M * = 8.70(±0.09) + 1.06(±0.10)log SFR, implying that star formation may have commenced at z > 10. No galaxies are found with SFRs much higher or lower than the past averaged SFR suggesting that the typical star formation timescales are probably a substantial fraction of the Hubble time. We report the first IRAC detection of Y 098 -dropout galaxies at z ∼ 8. The average rest-frame U − V ≈ 0.3 (AB) of the three galaxies are similar to faint z ∼ 7 galaxies, implying similar M / L . The stellar mass density to M UV,AB < −18 is ρ*( z = 8) = 1.8 +0.7 −1.0 × 10 6 M ☉ Mpc −3 , following log ρ*( z ) = 10.6(±0.6) − 4.4(±0.7) log(1 + z ) [ M ☉ Mpc −3 ] over 3 < z < 8.