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We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frameoptical) fluxes of 14 newly WFC3/IR-detected z=7 z_{850}-dropout galaxies and 5z=8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allowaccurate removal of contaminating foreground light, enabling reliable fluxmeasurements at 3.6 micron and 4.5 micron. None of the galaxies are detected to[3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detectionfor the z_{850}-dropouts and an upper limit for the Y_{105}-dropouts. Weconstruct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxesand fit stellar population synthesis models to derive mean redshifts, stellarmasses, and ages. For the z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1},(U-V)_{rest}=0.4, reddening A_V=0, stellar mass M*=1.2^{+0.3}_{-0.6} x 10^9M_sun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, andSSFR=1.7Gyr^{-1} are similar to values reported for luminous z=7 galaxies,indicating the galaxies are smaller but not younger. The sub-L* galaxiesobserved here contribute significantly to the stellar mass density and underfavorable conditions may have provided enough photons for sustainedreionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropoutshave stellar masses that are uncertain by 1.5 dex due to the near-completereliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), thestellar mass density to M_{UV,AB} < -18 declines fromrho*(z=7)=3.7^{+1.0}_{-1.8} x 10^6 M_sun Mpc^{-3} to rho*(z=8) < 8 x 10^5 M_sunMpc^{-3}, following (1+z)^{-6} over 3<z<8. Lower masses at z=8 would signifymore dramatic evolution, which can be established with deeper IRACobservations, long before the arrival of the James Webb Space Telescope.

the astrophysical journal. letters

Ultradeep Infrared Array Camera Observations of sub-L* z~7 and z~8  Galaxies in the Hubble Ultra Deep Field: the Contribution of Low-Luminosity  Galaxies to the Stellar Mass Density and Reionization