Star formation rates and stellar masses of z=7-8 galaxies from IRAC
  observations of the WFC3/IR ERS and the HUDF field
  We investigate the Spitzer/IRAC properties of 36 z~7 z-dropout galaxies and 3
z~8 Y-dropout 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 SEDs 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 beta~-2.0 to beta~-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 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 SFR according to log M* =
8.70 (+-0.09) + 1.06(+-0.10) log SFR, implying 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-dropout galaxies at z~8. The average rest-frame U-V=0.3 (AB) of
the 3 galaxies are similar to faint z~7 galaxies, implying similar M/L. The
stellar mass density to M_{UV,AB}<-18 is rho*(z=8) = 1.8^{+0.7}_{-1.0} x 10^6
M_sun Mpc^-3, following log rho*(z)= 10.6(+-0.6) - 4.4(+-0.7) log(1+z) [M_sun
Mpc^-3] over 3<z< 8.
