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

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We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z=7 z₈₅₀-dropout galaxies and 5 z=8 Y₁₀₅-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements 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 detection for the z₈₅₀-dropouts and an upper limit for the Y₁₀₅-dropouts. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For the z₈₅₀-dropouts, we find z=6.9⁺⁰.¹₋₀.₁, (U-V)rest=0.4, reddening Aᵥ=0, stellar mass M*=1.2⁺⁰.³₋₀.₆ x 10⁹ Mₛun (Salpeter IMF). The best-fit ages 300Myr, M/Lᵥ=0.2, and SSFR=1.7Gyr⁻¹ are similar to values reported for luminous z=7 galaxies, indicating the galaxies are smaller but not younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7<z<11. In contrast, the z=8.3⁺⁰.¹₋₀.₂ Y₁₀₅-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the stellar mass density to MUV,AB < -18 declines from rho*(z=7)=3.7⁺¹.⁰₋₁.₈ x 10⁶ Mₛun Mpc⁻³ to rho*(z=8) < 8 x 10⁵ Mₛun Mpc⁻³, following (1+z)⁻⁶ over 3<z<8. Lower masses at z=8 would signify more dramatic evolution, which can be established with deeper IRAC observations, long before the arrival of the James Webb Space Telescope.