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

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 5z ∼ 8Y105-dropout galaxies. The WFC3/IR depth and spatial resolution allow accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6μm and 4.5μm. None of the galaxies are detected to [3.6] ≈ 26.9 (AB, 2σ ), but a stacking analysis reveals a robust detection for the z850-dropouts and an upper limit for the Y105-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 ≈ 0.4, reddening AV = 0, stellar mass 〈M∗〉 = 1.2+0.3 −0.6 × 109 M (Salpeter initial mass function). The best-fit ages ∼300 Myr, M/LV ≈ 0.2, and SSFR ∼ 1.7 Gyr−1 are similar to values reported for luminous z ∼ 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+0.1 −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 2σ upper limit on the M/L(z = 8), the stellar mass density to MUV,AB < −18 declines from ρ∗(z = 7) = 3.7+1.4 −1.8 × 106 M Mpc−3 to ρ∗(z = 8) < 8 × 105 M 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.