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 z ₈₅₀-dropout galaxies and 5z ~ 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 μm and 4.5 μm. None of the galaxies are detected to [3.6] ≈ 26.9 (AB, 2σ), 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 z ₈₅₀-dropouts, we find z = 6.9^+0.1 _-0.1, (U - V)_rest ≈ 0.4, reddening A_V = 0, stellar mass langM*rang = 1.2^+0.3 _-0.6 × 10⁹ M _sun (Salpeter initial mass function). The best-fit ages ~300 Myr, M/L_V ≈ 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 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 2σ upper limit on the M/L(z = 8), the stellar mass density to M _UV,AB < -18 declines from ρ*(z = 7) = 3.7^+1.4 _-1.8 × 10⁶ M _sun Mpc^-3 to ρ*(z = 8) < 8 × 10⁵ M _sun Mpc^-3, following vprop(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.

Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #11563, 9797. Based on observations with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under NASA contract 1407. Support for this work was provided by NASA through contract 125790 issued by JPL/Caltech. Based on service mode observations collected at the European Southern Observatory, Paranal, Chile (ESO Program 073.A-0764A). Based on data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.