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We use a robust sample of 11 z~7 galaxies (z-dropouts) to estimate thestellar mass density of the universe when it was only ~750 Myr old. We combinethe very deep optical to near-Infrared photometry from the HST ACS and NICMOScameras with mid-Infrared Spitzer IRAC imaging available through the GOODSprogram. After carefully removing the flux from contaminating foregroundsources we have obtained reliable photometry in the 3.6 and 4.5 micron IRACchannels. The spectral shapes of these sources, including their rest frameoptical colors, strongly support their being at z~7 with a mean photometricredshift of <z>=7.2+/-0.5. We use Bruzual & Charlot (2003) synthetic stellarpopulation models to constrain their stellar masses and star formationhistories. We find stellar masses that range over 0.1 -12x10^9 M_sol andaverage ages from 20 Myr to up to 425 Myr with a mean of ~300 Myr, suggestingthat in some of these galaxies most of the stars were formed at z>8 (andprobably at z>~10). The best fits to the observed SEDs are consistent withlittle or no dust extinction, in agreement with recent results at z~4-8. Thestar formation rates (SFR) are in the range from 5-20 M_sol/yr. From thissample we measure a stellar mass density of 6.6_{-3.3}^{+5.4}x10^5 M_sol/Mpc^3to a limit of M_{UV,AB}<-20 (or 0.4 L*(z=3)). Combined with a fiducial lowerlimit for their ages (80 Myr) this implies a maximum SFR density of 0.008M_sol/yr/Mpc^3. This is well below the critical level needed to reionize theuniverse at z~8 using standard assumptions. However, this result is based onluminous sources (>L*) and does not include the dominant contribution of thefainter galaxies. Strikingly, we find that the specific SFR is constant fromz~7 to z~2 but drops substantially at more recent times.

The Stellar Mass Density and Specific Star Formation Rate of the Universe at z~7