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 similar to 7 z(850)-dropout galaxies and three z similar to 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 similar to 7 galaxies are best characterized by substantial ages (>100 Myr) and M/L(V) approximate to 0.2. The main trend with decreasing luminosity is that of bluing of the far-UV slope from beta similar to -2.0 to beta similar to -3.0. This can be explained by decreasing metallicity, except for the lowest luminosity galaxies (0.1L(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 similar to 7 star-forming galaxies correlates with star formation rate (SFR) according to logM* = 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 similar to 8. The average rest-frame U-V approximate to 0.3 (AB) of the three galaxies are similar to faint z similar to 7 galaxies, implying similar M/L. The stellar mass density to M(UV,AB) < -18 is rho*(z = 8) = 1.8(-1.0)(+0.7) x 10(6) M(circle dot) Mpc(-3), following log rho*(z) = 10.6(+/- 0.6)-4.4(+/- 0.7)log(1 + z) [M(circle dot) Mpc(-3)] over 3 < z < 8.