Abstract

We study the Spitzer Infrared Array Camera (IRAC) mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z similar to 7 z(850)-dropout galaxies and 5z similar to 8 Y(105)-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6 mu m and 4.5 mu m. None of the galaxies are detected to [3.6] approximate to 26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection for the z(850)-dropouts and an upper limit for the Y(105)-dropouts. We construct average broadband spectral energy distributions using the stacked Advanced Camera for Surveys (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) approximate to 0.4, reddening A(V) = 0, stellar mass M*> = 1.2(-0.6)(+ 0.3) x 10(9) M(circle dot) (Salpeter initial mass function). The best-fit ages similar to 300 Myr, M/L(V) approximate to 0.2, and SSFR similar to 1.7 Gyr(-1) are similar to values reported for luminous z similar to 7 galaxies, indicating the galaxies are smaller but not much 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(-0.2)(+ 0.1) Y(105)-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 M(UV, AB) -18 declines from rho*(z = 7) = 3.7(-1.8+1.4) x 10(6) M(circle dot) Mpc(-3) to rho*(z = 8) 8 x 10(5) M(circle dot) Mpc(-3), following alpha (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.