The Stellar Mass Density and Specific Star Formation Rate of the
  Universe at z~7
  We use a robust sample of 11 z~7 galaxies (z-dropouts) to estimate the
stellar mass density of the universe when it was only ~750 Myr old. We combine
the very deep optical to near-Infrared photometry from the HST ACS and NICMOS
cameras with mid-Infrared Spitzer 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 and 4.5 micron 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 (2003) synthetic stellar
population models to constrain their stellar masses and star formation
histories. We find stellar masses that range over 0.1 -12x10^9 M_sol and
average ages from 20 Myr to up to 425 Myr 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 (SFR) are in the range from 5-20 M_sol/yr. From this
sample we measure a stellar mass density of 6.6_{-3.3}^{+5.4}x10^5 M_sol/Mpc^3
to a limit of M_{UV,AB}<-20 (or 0.4 L*(z=3)). Combined with a fiducial lower
limit for their ages (80 Myr) this implies a maximum SFR density of 0.008
M_sol/yr/Mpc^3. This is well below the critical level needed to reionize the
universe at z~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~7 to z~2 but drops substantially at more recent times.
