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We use a robust sample of 11 z ∼ 7 galaxies (z 850 dropouts) to estimate the stellar mass density (SMD) of the universe when it was only ∼750 Myr old. We combine the very deep optical to near-infrared photometry from the Hubble Space Telescope Advanced Camera for Surveys and NICMOS cameras with mid-infrared Spitzer Infrared Array Camera (IRAC) imaging available through the GOODS program. After carefully removing the flux from contaminating foreground sources, we have obtained reliable photometry in the 3.6μm and 4.5μm IRAC channels. The spectral shapes of these sources, including their rest-frame optical colors, strongly support their being at z ∼ 7 with a mean photometric redshift of 〈z〉 = 7.2 ± 0.5. We use Bruzual & Charlot synthetic stellar population models to constrain their stellar masses and star formation histories. We find stellar masses that range over (0.1-12) × 109 M ⊙ and average ages from 20Myr to 425Myr with a mean of ∼300 Myr, suggesting that in some of these galaxies most of the stars were formed at z > 8 (and probably at z ≳ 10). The best fits to the observed SEDs are consistent with little or no dust extinction, in agreement with recent results at z ∼ 4-8. The star formation rates (SFRs) are in the range from 5 to 20 M ⊙ yr-1. From this sample, we measure an SMD of 6.6+5.4 -3.3 × 10 5 M ⊙ Mpc-3 to a limit of M UV,AB L*) and does not include the dominant contribution of the fainter galaxies. Strikingly, we find that the specific SFR is constant from z ∼ 7 to z ∼ 2 but drops substantially at more recent times.
González et al. (Thu,) studied this question.