Stellar Properties of z ≳ 8 Galaxies in the Reionization Lensing Cluster Survey

Measurements of stellar properties of galaxies when the universe was less than one billion years old yield some of the only observational constraints on the onset of star formation. We present here the inclusion of Spitzer/IRAC imaging in the fitting of the spectral energy distribution of the seven highest-redshift galaxy candidates selected from the Hubble Space Telescope (HST) imaging of the Reionization Lensing Cluster Survey. We find that for six out of eight HST-selected z ∼ 8 sources, the z ∼ 8 solutions are still strongly preferred over z ∼ 1-2 solutions after the inclusion of Spitzer fluxes, and two prefer a z ∼ 7 solution, which we defer to a later analysis. We find a wide range of intrinsic stellar masses (5 × 10⁶-4 × 10⁹ M_⊙), star formation rates (0.2-14 M_⊙ yr^-1), and ages (30-600 Myr) among our sample. Of particular interest is A1763-1434, which shows evidence of an evolved stellar population (∼500 Myr) at z ∼ 8, implying that its first generation of star formation occurred <100 Myr after the Big Bang. SPT0615-JD, a spatially resolved z ∼ 10 candidate, remains at its high redshift, supported by deep Spitzer/IRAC data, and also shows some evidence for an evolved stellar population. Even with the lensed, bright apparent magnitudes of these z ≳ 8 candidates (H = 26.1-27.8 AB mag), only the James Webb Space Telescope will be able to exclude the possibility of abnormally strong nebular emission, large dust content, or some combination thereof, and confirm the presence of evolved stellar populations early in the universe.