Abstract

We use a robust sample of 11 z similar to 7 galaxies (z(850) dropouts) to estimate the stellar mass density (SMD) of the universe when it was only similar to 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 3.6 mu m and 4.5 mu m IRAC channels. The spectral shapes of these sources, including their rest-frame optical colors, strongly support their being at z similar to 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) x 10(9)M(circle dot) and average ages from 20 Myr to 425 Myr with a mean of similar to 300 Myr, suggesting that in some of these galaxies most of the stars were formed at z > 8 (and probably at z greater than or similar to 10). The best fits to the observed SEDs are consistent with little or no dust extinction, in agreement with recent results at z similar to 4-8. The star formation rates (SFRs) are in the range from 5 to 20 M-circle dot yr(-1). From this sample, we measure an SMD of 6.6(-3.3)(+5.4) x 10(5) M-circle dot Mpc(-3) to a limit of M-UV,M-AB -20 (or 0.4L(z=3)*). Combined with a fiducial lower limit for their ages (80 Myr), this implies a maximum SFR density of 0.008 M-circle dot yr(-1) Mpc(-3). This is well below the critical level needed to reionize the universe at z similar to 8 using standard assumptions. However, this result is based on luminous sources (>L*) and does not include the dominant contribution of the fainter galaxies. Strikingly, we find that the specific SFR is constant from z similar to 7 to z similar to 2 but drops substantially at more recent times.